In addition, Amigas had three types of RAM to begin with - Chip Mem (shared between custom chips and CPU), Slow Mem (exclusive to the CPU, still IIRC as slow as Chip Mem) and Fast Mem (exclusive to the CPU and significantly faster).

And just disabling the upper memory in order to be able to use the PCMCIA slot is a really lazy solution. Kinda typical for Commodore, though. 3rd party vendors offered better designs for their memory expansions.