Provided you give them a new TIS, I see no reason why not, unless you have numbering issues (example-our bank uses a system where the first number of the account tells you the type-checking, MMA, or savings). If you do have number issues, you would want to close the savings and open a new checking, then dump their funds in that account....of course, you will have to get them to come in a sign new signature cards if you go that route....
_________________________
I can't herd the cats anymore, so I just set up the electric fences and let them fry when they stray out of bounds.