If you want to switch characters out on the fly without having 5 to the power of 5 Character Sets, design your Character Set and its Key Maps (etc) so that you can just switch out a Character in any given Slot.

On my old Pal + 4 DK team, I would swap out any of the 3 DPS DKs for a shaman, rogue, druid, etc. All I had to do was modify the Character Set real quick, because I designed my setup so that this would work. If I had more characters on my current server I could do the same thing with my 5 Paladins. (Maybe if I level up my DKs for PVP I can now do the infamous DK + 4 Pal)

This is achieved by removing any assumptions about what character is in each Slot. I make heavy use of Action Target Groups to make sure that if it's something that only ret paladins should do, only they do it. Or if it's only for rogues, only they do it, and so on. So if you have 5 shamans, instead of just putting "all" or "all other" and assuming that they're all shamans, you would specifically put "all shamans" or "other shamans" instead. Then if you put in a druid or mage with your 5 shamans for example, you can specify something for them to do instead of the same thing the shamans do.

This saves me the trouble of micromanaging dozens of character sets that each have one character changed in it, and maximizes my reuse of existing configuration.