Am I being discriminated against at work?

That is not illegal discrimination by any definition.

It is not about fairness. You can ask anything you want. You cannot make them "do" anything.

All discrimination is not illegal. Only that based on age, race, religion, etc.

It is not only 100% legal, but 100% PROPER to discriminate on the basis of your ACTIONS.
You have NO IDEA if the others have been called in for similar issues.
You also have NO CLUE if they actually consulted the manager BEFORE doing the things you think are unfair.

The problem could be what you have not seen between the boss and the other employees. It could be that the boss has already spoken to or made arrangements with the other employees concerning what appears to you them "coming and going as they please". But it could also mean that your specific tasks are much like a central cog in a wheel where your task puts together the random work the others have done. For example, in a "big box store" department, the others could be assigned to clean aisles, replace clothing on the racks and sort, but if you are the person hanging up advertising signs and making sure the advertising and products displayed are correct, your task, although able to be completed part-time hours, still must be done according to a schedule where hanging clothes and folding jeans and t-shirts can be done by any one at any time. But, you are also not a slave! You have the freedom to let them growl but you can ignore the growls and not accept any threats against you! If they make threats, explain this to them -- you are NOT a slave and this is just a job where you complete their tasks, and yelling at you in inappropriate behavior and poor management from THEM. You have the right to take your time as you need and you have the right to manage your work hours as you need and the only thing they can really complain about is if your work times are conflicting with regular deadlines, where you are not completing a task in time to meet a deadline because you have to leave at an unreasonable hour. They can make unpleasant noise about you if they choose, but you can leave the workplace and go outside and leave their unpleasant-ness behind you as you go. I know of a lot of people who figure out ways to leave work grumbles at work and approach home time with family as "their time" that work does not invade. They have the right with Freedom of Speech to voice their displeasure, You have the right to not like what they say but also to exit the building and walk away. Try letting it go a little longer until you move and think of ways in your next job to work with your new managers a more reasonable agreement and good luck to you!

"Can I say anything to them to make the treat me fairly?"

No. But you can clarify what your employer's expectations are and you can ask for those expectations to be given to you in written format. You can also ask if those are the same expectations for other employees as well, but it's really none of your business what someone else's arrangement is with their employer.

Discrimination is perfectly legal unless it's a very specific circumstance that is prohibited by law such as race, gender, etc.

Most employees would be thrilled to have an employer who lets them bounce between full-time and part-time, have flexible hours, and arrange their work around their personal lives. I suggest you not be too indignant about the whole thing. They might decide that bending the rules has become a pain in their butt and "fair" means that everyone's flexibility goes away in favor of a more rigid set of rules.

I don't think you're being discriminated against as much as they're being hypocritical. I personally would not go whining to the boss saying that "Well, how come THEY get to do what they want?" because that might make me seem childish.
What I would do, though, is have a chat with whoever gave you a hard time, with as many people involved as necessary who agreed to the original transition from full-time to part-time, and say you're confused about what their policy is because you were told X but now they're saying it should have been Y.
I'm not clear if you're moving away from this job ultimately, if so I'm not clear why it matters, but if not, then yes, I'd say something...but with an eye towards clarification, not trouble making.

Yes but there is nothing illegal about it. Clearly you're expected to know better and instead choose to act like those who don't.

It doesn't matter what anyone else does - only what you do, and it sounds like your employer doesn't consider you of any value.

Most folks who move manage to do it around their full time work schedule.

Yes