I reused a dorm once but after having one of those mysterious "OMG! Scratchy icon thought bubble! Eek!" spots now I always bulldoze the dorm when my Sims leave and replace it with a new copy from the lot bin. I have been using
this dorm without a problem since my founder started Uni, but when the bills stopped coming I downloaded a couple more to see if it was just that one lot.
ETA: Problem solved with the addition of a new CAS dormie, just as Lord Darcy suggested. Thanks again!