Why can't defstructs be redefined?

Anyone know this area of the compiler? It's very frustrating during development. Seems we ought to, at least, be able to blow away all traces of the defstruct, ignoring existing structures and redefine it. My attempt to do that the obvious way - removing the system::structure-definition property from the symbol plist of the struct name - allows the redefinition but sometimes errors out when I use the new structure.

Anyways, I'd like to fix this, and any info would give me a head start.

Thanks, Alan