What are the problems that might come up if one of the persons named on a joint POA wants to move out of state?

If your POA appoints you to act 'jointly and severally' then there's no problem at all. If it's plain 'jointly' - as in, requires both your signatures to any document, for example - then still not problems as such, but a pain in the neck and potentially quite profitable for courier companies like UPS if you're ever in a hurry to get stuff done.

Is it too late for the elderly parent to revise his/her POA?

There shouldn't be any problem if POAs were set up so person could act independently.