Just wanted to write in with a feature request (or maybe it already exists?). It would be great to move some problems from one set to another, once the source set has already been assigned. For example, I was overly ambitious and released a problem set with all the material I hoped to cover in the next week. I didn't end up covering the material for the last several problems, and would like to move them to next week's problem set, without losing the work that a few students may have done on these problems already. Alternatively, if such a capability existed, I could just alter the due date for those specific problems (i.e. grant an extension for just part of the problem set).
Hopefully as I gain more experience, I'll just learn to be more realistic when planning ahead! :-)