simply put this is impossible to do 100% effectively using GBC hardware because file size and processor mismatch. according to wikipedia a GBC runs a two mode 4-8Mhz, 8 bit processor. the GBA runs at 16Mhz and uses a 32 bit processor. The processor chips are extremely different and on the GBA hardware they both exist with the original GBC Z80 as a Co-processor with the GBA's arm processor.
GBC games use 8MB max
GBA games use 32MB max
I did not say it was impossible, however it would require a complete software port and many sacrifices would have to be made espeacially because you are going from a 240x160 pixel display with 512 colors to a 160x144 pixel display with 56 colors.
The memory issues could be worked around...
A fair analogy would be a port of Xbox 360 to NDSi which happens all the time, however it fundamentally changes the game to the point where its not even the same game and requires full development studios to do it.