You may or may not have a fair lending issue. Look to the discussions on disparate impact and ask yourself if the effect of this change will adversely impact protected classes in either your applicant pool or your market population as a whole. On the surface, it probably won't - but since I just sat through 3 days of fair lending training, I'm not taking anything to chance.
I agree that you should include this in your initial loan contract. State law will govern whether you can make it a retroactive change (I doubt it), and if so, how you'd go about it.
Fair lending aside, you shouldn't have any problem with this on a going-forward basis - provided that kind of fee is allowable in your state.
Opinions are Bartman's, not those of my employer. "A noble spirit embiggens the smallest man."