Congress can change anything it wants to. Remember the fast track trade deals. Congress voted on fast track, so the rules were nothing can be changed in that one trade deal. Each one is different. But I don't see any troubles with this deal. The democrats like it because it is very pro union, so there will be gripes about this or that, but it will be approved.
I don't think this is considered a treaty, but the Senate has changed treaties all the time. They will vote on side provisions, etc.