J.J.
Hello and congrats on your son!!
Yes she will go to bed again without breaking your heart. She is going through a lot of changes and probably senses you are too. She could be going through a growth spurt or teething. She could be just an overwhelmed two year old..they all get that way sometimes.
I have a 7 year old who fights going to bed some nights and others, just like you said, will just go to sleep willingly. She may be having a slightly harder time making this adjustment into the big girl bed, but she's adjusting. Don't worry, it does get better and the crying will get less and less.
As for potty training...if she can tell you she has to go in one way or another it would be a good idea to start. I waited until my first could communicate to me somehow she had to go and it worked well.
Don't worry mom, she's learning how to adapt to change, it's a good thing!!!!!!