Note: Feel free to correct me if I'm getting the hierarchical order wrong. It's hard to tell which droid model is "better" or simply a different specialization.
I'm thinking from the ground up, wondering if a regular B1 has ever been promoted to a new type, such as becoming a pilot or OOM commander droid after surviving x amount of battles, or proving that they're either glitch free or that their glitch is useful, and then possibly their "consciousness" transferred to a B2 and beyond. I know B2s aren't as smart(1), but they are much stronger and more durable, so this could be seen as an improvement from a robot's perspective.
One droid in particular that was able to quickly adapt to an unscheduled change in development comes to mind as a potential candidate for immediate advancement.
So has this ever happened? If not, does any system for personal battle droid advancement even exist?
(1) At least initially. If fitted with a previously proven "brain", the tactical function of a B2 could see an improvement, and would make this kind of system make more business sense.