I haven't had this happen before, mainly because we make our own content most of the time.
For whatever reason, I've got two families that are both in the same category, but I can't switch between them. If I select one and switch to the other, I get the wonderful can't make type error.
1. All the other instances of that type are fine.
2. I can place new instances of either family, any type.
3. I've checked all the instance parameters to make sure they can be set the same across both families without breaking it.
Based on that I believe both families are fine (not breaking or anything).
That leaves me with this thought...
1. Since these are two very different families with very different parameters, Revit can't figure out what values go where when you change families and thus gives that wonderful error.
2. Since one family has a bunch of shared nested components, and the other has a bunch of non-shared nested components, it is having a hissy fit switching families out that have different nesting situations.
I think #1 is more likely, but I'd like to know if there is a "rule" that can help me change the minimal amount of parameters to let me flip flop families. Does it only matter with Type parameters? Does it matter if one family has a paramater but the other doesn't? Does it matter is one parameter is instance in one family and type in another? Does it matter if one parameter has a formula in one family and not in another?
Thanks!