Covak whipped out his/her/its Personal Electronic Thing, summoned a
BellBot to 83Nbb.95047$DZ.89184@news04.bloor.is.net.cable.rogers.com ,
leaving his/her/its contact info (covak@rogers.no.spam.com) with the
deskbot, and proceeded to say:

Different voltages in the carts. If you notice, there's a switch inside the
cartridge slot that the GBA carts miss with their indented edge. This
activates GBC mode. If you want to use GBA flash carts to use GBC roms, you
need a REALLY unsightly converter.

No you need to use what they call a GameBoy Bridge. Because GBA games run on
a different voltage to the GB/GBC games from what I understand. Also the F2A
is the only one that I know of that has this bridge. I personally have a F2A
myself and I brought the bridge when I brought mine. But I have not tried to
put my GB/GBC games onto the flash cartridge yet. Also I was reading on a
message board that there is a bit of mucking around to get both GBA and
GB/GBC games to work on the one flash cartridge. But again I have not tried
it yet so I cannot comment further. One of these days I will get around to
playing with it more. So maybe someone else on this NG might be able to help
you further with putting GB/GBC games on the same cartridge as they GBA
games.