My approach to this is:
You can add letters, but it's more difficult to reassign since once submitted the program may have downloaded them - in which case they will have the letter. Thus it's better to only assign the letters you are certain about initially and then add the other letters later. The caveat to this is that until you assign the letters to at least one program your school can't upload them.
What I did is I assigned all letters to one of my programs (one that I didn't care as much about and which required 4 letters.) To the rest of the programs I assigned the two letters I was certain about using. Once I figured out about the other 2 letters I then added them to the rest of the schools.
