I assume that you are refering to the roof edge flashing and that it is nailed to the underside of the roof that you do not want disturbed..
This does present quite a challenge because in most cases when you are replacing the facia board, the roof edge has to come off. The roof edge creates a smooth transition from the roof to over the facia board. It retards rot and channels the water over this board into the gutters if you have them.
With great care you can carefully bend up enough to allow the facia board to come up with the old roof edge still fastened. Then once the new facia board is installed, just bend it back down. this is the hard way, but in your situation, the only way I know.
You must have the roof edge there for protection of the new facial board.
Now, if there is a way to keep the old roof edge there and enough space to install a new roof edge into the slot, you can do this. I would then suggest to get a new roof edge section that is bigger and thus will fit over the old.