AAUP standards for timing of recruiting/resigning a faculty position: Do they really mean anything?
I have seen faculty leave or take positions at all kinds of times during, before, or after the semester or even right after (or during!) sabbaticals.  The AAUP guidelines are nice, but, ultimately, everything depends on the schools involved.

If you are concerned about ethics, I would recommend being upfront with both departments (the one you are leaving and the one where you will be hired) and coming up with something that leaves both of them satisfied.