Verb that means to change oneself for someone else?

I would say "compromise," such as, "He compromises himself for her" or even "He is compromised by her."

If it starts with P, the only thing I can think of is "placate," such as, "He placates her." That means that he behaves in a manner that satisfies her demands. Whereas it doesn't start with P, the word "appease" has a very P sound about it and might also be used.

Another possibility is "patronize." However, that has the added connotation of doing so in a condescending manner. It refers to changing oneself for someone else but in a way that obviates one is doing so insincerely, while looking down one's nose at that someone else, and only in order to achieve some other end. It's like when you return something defective to the store and can tell the clerk doesn't believe you when you say it was already broken when you opened it, so you say, "It really was broken," but the clerk responds with a big, fake smile, "Sure it was. As they say, 'The customer's always right!' So go ahead and pick another," making you want nothing more than to punch that clerk in the head. The clerk is still changing himself for the customer but doing it with utmost condescension, by patronizing.

Pandering? Humoring people to fit in.

"pandering" doesn't mean changing oneself. It usually means saying things that you think other people want to hear.

Accomodate