You will most definitely be able to change the characters name.
Yoshi-P said it is technically possible without any problems (iirc in Live Letter III or IV, first or second question),
though they would limit the number of letters (max. 10 prename, max. 10 surname).
Old names longer than that will remain as they were.

I said most definitely because they didn't confirm it 100 % . . . yet.