Is it possible to turn from an Introvert into an Extrovert in a day?

Well, it's great you that have the ability to be loud and extroverted. Pretend like it's a muscle. Go ahead and flex those muscles whenever you want to or whenever you feel like it.

On the other hand, being introverted might be your natural state, which is not bad at all. Now, if you're introverted and you don't like talking with others, you aren't being bold whenever you need to, you're not standing up for yourself, etc. that's bad.

Go ahead and allow yourself to be introverted if it's easiest for you. You want to be sort of a hybrid though. You should have a strong, powerful "extrovert muscle" that you can summon instantly. You'll use it whenever you have to stand up for yourself, be bold, raise your voice, say "no", etc.
Then, you want your natural, free flowing introverted state. You don't feel the need to be in the presence of others and you talk in a low voice, yet you are aware of your surroundings and the people you interact with. You speak slowly with confidence, and your thoughts, words, and actions are all aligned. You are happy with yourself, you do not care what others think or say, but you're curious about them and about the world. You breathe slowly, act calm and focused in all situations, and feel a great power within you. You are your own man, nobody can make you flinch, and when you need to, you have the ability within you to be loud, aggressive, and loose.

Hope this helps.

you realised that you possibly might not have to see some of those people anymore that might have been judgemental or nasty in the past. That's enough to spark life into anyone

introverts are sneaky

No absolutely IMPOSSIBLE. Switching from introvert into extrovert is not possible in a day. Besides, as long as you are introvert, that would be you for the rest of your life. We are not mechanical robots that a change in microchips up in the head will change our view over night. Completely preposterous.

Even if you say you are confident and kinda loud, it will retort back to your original form "introvert". When something in your life hit the fan or downside like tragedy or lonely moments. So your confident is only just for a few minutes or hours, just like the story of Dr. Jekyll and Mr. Hyde. The fictional story goes that the doctor problem is how to convert himself to be extrovert and have good PR or public relation to almost everyone including the desire to the one he loves. So he cook up some formula to do that, the only problem is he made a wrong calculation of the formula and he became a monster instead. So in the morning he is the well mannered doctor but at night the leaping creepy monster Mr. Hyde who do the killing. But the spell were off when the morning comes. Just like your being confident, it also wear off in time. You cannot change who you are. Just like if you are good, no matter what you do or what other say to you, you are good. But if you are bad, no matter what you do also, you will always be bad.

well possible, but not recommended.
normally, it takes about a week/month to change, and I don't recommend rushing it through.
making the sudden changes may make you feel uncomfortable in the long run, so think carefully before you choose. you don't necessarily have to be an extrovert, you just need to be comfortable with yourself and live your life with other people in a way that is normal and acceptable to others.
a sudden change from a personality of introvert to extrovert could/might cause problems, say you might not get used to the extrovert lifestyle, and the other people may not get used to it, so it takes time. that's why I mentioned not to rush it through.
but at the end of the day, it's your choice. you can choose to do it in a day, week, or month, but I recommend doing it for a longer duration, say a week/month. you don't necessarily have to rush it.

Here’s are six steps you can take.
1. Know thyself. Introverted people are more likely to be good listeners and then process information, while extroverts are the ones putting it out there and picking up their information from external triggers. If you can recognize what your natural state is, you’re in a better position to understand your behaviors and practice assimilating them with your opposite behaviors to achieve the full range of your capabilities.

2. Be receptive. It’s tough to adopt a personality trait that you see as undesirable. Too often introverts will stereotype extroverts as shallow and chaotic. On the other hand, a lot of extroverts will wrongly pigeonhole their introverted counterparts as uptight, aloof and boring. Perhaps you’re having trouble coaxing out your less dominant side because you subconsciously don’t want to or are afraid of doing so.

You can overcome this mental barrier by making a list of all the introverts/extroverts in your life whom you like, love, respect or admire. List the qualities that you like in them and you’ll find that a lot of those characteristics stem from their dominant side. In the process, you may realize that introversion (or extroversion) can’t be that bad after all.

3. Do the opposite. You can’t change behavior patterns overnight, but you can practice behaving differently until it comes more naturally to you. For example, extroverts can substitute their usual attention-grabbing attire for more practical clothing at least one day a week. They can also try listening to more soothing, chilled out music than the upbeat tempos that normally fire them up. In contrast, introverts could have a go at drawing attention to themselves and their workspace by putting photos on the wall or flowers on the desk. A bowl of grapes or hard candies on the desk will also draw more co-workers than usual towards the introvert – who can resist a free snack?

4. Give a performance. Many actors, impressionists and other performers are actually quite introverted. When they step into the character, however, they effervesce just like extroverts do. Robert De Niro, for example, is famously known for enjoying his privacy yet, as one of America’s greatest character actors, he’s able to step into practically any role and take on the personality traits of the character.

Even though it’s not who you inherently are, it is possible to play the game and put on a performance as an extrovert (or introvert). Think back to your school plays or glee club and what you did to really get your teeth into a role or musical number. Don’t think of it as being fake, but rather as a case of “act it until you believe it.”

5. Watch and learn. If you ask introverts and extroverts to do the same management exercise, you’ll notice huge differences in how the two groups approach the task. In the introvert group there will be times when no one is talking and everyone’s just listening, watching, sitting and writing notes. It will come across as a disciplined environment. In contrast, the extroverts are likely to be talking over one another, standing up or drawing on whiteboards in what amounts to a disorganized environment.

We don’t notice these extremes in other contexts because most groups of people are a mix of introverts and extroverts. Whether you’re an introvert or an extrovert, there will be people who are the opposite of you in your social group or your team at work. Observe what these people do differently from you and learn from them.

6. Unearth hidden depths. What if the introvert or extrovert is not you, but a member of your team at work? When I chair meetings and know the people around the table, I watch introverted colleagues closely and make an effort to include them, because they’re not naturally going to speak up. Instead, most of the noise will be coming from extroverts.

An effective leader will offer protection to the quieter participants and remind the more outgoing ones to be inclusive. Give introverts time to speak and encourage the extroverts to listen more. The next time someone is about to interrupt or talk over someone else, ask them to wait and encourage the other person to keep talking. You and others at the meeting may be pleasantly surprised by how much they have to say.

Well,sure it's possible! You are living proof,and way to go!!!