I have a rather simple threadless headset in aluminum frame, and forks. It's creaking quite a bit, I think I'll replace the bearings and races. It has "retained" bearing rings (semi-loose) and cup races. I was wondering if it's possible to replace these with a sealed type bearing set?
Yes, it is possible. The headset compatibility is determined by steerer tube and head tube dimensions. The type of bearings inside does not really matter, and the difficulty of getting spare parts for sealed bearings or maintaining loose ball bearings are both exaggerated.
When replacing the headset, check the condition of the frame: if the creaking is because of deformed head tube, a new headset will not help.