I asked this question in the "simple question" thread, although it's not really a simple question.
Here's the situation:
Beguiler 2 / Wizard 4 / Ultimate Magus 10, with Practiced Spellcaster (Beguiler). This causes the 3 single-progression UM levels to push the wizard side each time. (as an aside, it's best to delay beguiler-2 until right before UM-7).
Anyway, at some point after UM-7, this character retrains away Practiced Spellcaster (Beguiler), changing his caster level on the beguiler side.
Under normal conditions, if you retrain a feat that provides a prereq to another feat, the second feat is just disabled, dead weight. If you retrain weapon focus, it means you don't get to use weapon specialization. But this is different, retraining a feat that affected decisions you made while gaining other levels.
According to all the RAW I can find, it seems like a valid (ab)use of the retraining rules. It's definitely an AYDM situation, and I'm quite frankly afraid to even ask (I've already got a rep at my table as an optimizer). Still and all, it seems to work.