This isn't specific to his being trans, but keep in mind: you broke up for VERY good reasons. This person hurt you so badly you ended your marriage and took steps to stop him seeing your daughter unsupervised. I'm sure none of that was easy, and that you must have given him many second chances and tried everything you could before leaving.
So, remember nothing has changed, and you cannot go back to where you were. Best case, your ex really has discovered something life changing and will - eventually, with extensive therapy and a lot of work - overcome some of his issues. But even if he's on the level, it's a long, long process and he's only at the very beginning. If it's even possible for you to trust him on any level again, it will take years. And you have no obligation to even try. I think this changes nothing for you: you still continue with the divorce, you still keep your daughter safe and, since he's given you some startling new information, you stay wary.
Imagine if he'd instead told you he discovered he was an alcoholic and now he'll get help. It's a change, and it seems positive on the surface - but you have no idea if his self-diagnosis is correct, if he'll stick with his plans to quit, if he'll be successful, etc. It will be years before you know. And even then, the doubts and mistrust and the memories of the painful past are still there. The idea that there's some magical explanation or cure or change - it's a fantasy. You're not even sure you believe what he's telling you. Be very wary, but go ahead and get on with your life. You can wish him luck, but you can't trust him.