First things I consider for powder choice are:
1. "What do I already have the most of"
2. "What do I know is likely to be in stock in the future"
3. "What powder am I already using so I can stock it in bulk"
Now, for most loads I have a few powders that fit the bill. H1000, 4064, Unique, LilGun, those types. So usually I'll try to start load development with those. Now, to the question about a change in powder, my goal is that once I have a good starting load, I go back to the book, and ask the same questions but making sure the answer is a secondary powder. Then I'll try to do it a third time.
My goal through the development process is to find 1 load in a primary powder that works really well, somewhere near the 3/4 range of the scale. (For instance, if the book data for 4064 says 60-70gr, I'd love my accuracy node to fall around 67.5). That way I have a large margin of safety, and when I work on load B and C, the ranges usually overlap in velocity. That gives me a lot more ability to tweak the powder charge so that I can get closer to the same velocity and the same POI.
If everything goes well, I'll wind up with three loads from different powders that all come close to the same POI so that I'm well insulated from shortages.
That said, I keep probably 30ish types of powder around for development and testing since sometimes I'll throw a wrench in my process and start based on recommendations for a given caliber or model of firearm. So I'll have a lot of the 1 pounders of those, while my primaries are bigger jugs.