Game Problem: Can i change an emulator rom without starting a new game?

im trying to change my Pokemon: leaf green rom; but can i change the rom w/o starting a new game? the issue in the game is in some areas with multiple pokemon, i see the same one, same level, same gender, and i know its not a coincidence, even in the safari, i see only 2 kinds of pokemon with same gender,and level

it would be great if someone could answer.

You have to ask yourself: can you do what you want on the real console?

Therein lies your answer.

The GBA doesn't have a memory card, so no, you can't.