I can't remember which file replaced which.  But I think the file you want is in /admin/Users/Profile.ascx.  However, by changing anything here you change it for *every* portal in that host environment, which may or may not matter depending how your are using DNN.

I had a similar issue with a DNN Product.  Our customers wanted to customize the type of information they collected, as well as change the way it looked.  Fortunatly two customers asked for it at the same time, so we were able to see the problem with modifying this file.  I used (and am in no way affiliated with) Dynamic Registration 2.something... I bought the source which ended up being a good idea because a lot of things were hardcoded, but in comparison to what I would have *had* to write this thing saved me 600-1000 hours easily.  I had a complex situation though, people that were unwilling to accept anything but perfect, if your customer(s) are a little more lax, this may answer all your problems without any modification.  I know newer versions exist than the one I had.  It was a good price too for the time saved and what it accomplished.  Again, I'm not promoting it, just saying how I dealt with a situation and it worked out pretty well. 

HTH,

Chris