The loader and patcher needed to run pirated ROMs are unique to each card (except in the case of clones like FAST mentioned), so that's why you can't just swap around different firmwares. If you're only using the card for homebrew and media, then you may be able to use the firmware from another card and you can definitely use homebrew menu systems as replacement firmwares. This would work with most cards, I think, including the R4 thanks to r4crypt; but with some cards like the CycloDS Evolution you can't replace the firmware with anything but another official firmware, so the only thing you could do is set up your alternate menu to autoboot whenever you start up the card.
Since the loader for the Acekard RPG (and Acekard 2 is the same, I guess?) is open source, it's possible to write all new firmwares around it or drop it into existing open source menu systems so that you'd be able to load commerical ROMs with them. That's a unique situation for the time being, though. It'd be nice if more cards went open source.
...word is bondage...