Naaaah, I already fixed that on all of my games (and did so a year ago for Crystal, even, because the battery had already died).. It's preventable without losing your savegame
Just hook up a new battery in parallel using some small wires, then take out the old battery (it's tacked onto the tabs so it'll be hell to pry it loose without messing up the tabs; use a jeweler's flathead screwdriver) and place a new button cell battery (either CR2016 or CR2025 will fit in the GB/GBC game cartridges if you decide to use tape/hot glue, CR2032 if you can cram it), then take off the temporary battery and there you go, game save file preserved and a brand new back-up battery!
Do it before it's too late because it'll be irreversible when it disappears!
On another note, the GBA games (Gen III) weren't the only awesome ones eh, think about Gold/Silver/Crystal! Together with the Gen I games I think Gen II was the most fun to play overall.Edited by RevZ - 5/3/12 at 1:03am