Chances are you may lose your saves when you remove the battery, but that's not always the case. Not exactly sure what it depends on (maybe depending how much juice is still left in the battery your replacing and how quickly you can get it done) but I have swapped out the batteries on a few games, and couple of times to my surprise, the saves were still intact. Could have something to do with the energy from the battery not completely de-charging from the board before the new battery in fixed in?
Either way, I wouldn't count on it, but its possible. With that being said, swapping out a dead battery in an NES/SNES cart with a clip was the first time I ever used a soldering iron. And after a simple youtube tutorial and gathering the few things I needed, I was good to go. It's pretty damn basic and not much is needed. And from there I've gone on to do more cmplicated things like installing region switches in my genesis, mod chips in my Saturn, audio mods for my NES and various Neo Geo MVS repairs with very little worry and issues.
It's a good starting point to get anyone familiar with the basics on soldering. And if your worried about damaging the game (which isn't exactly as fragile as some might think) you can always try a shitty sports game as suggested. All in all, a very simple project to do. But I guess this kinda stuff might not be for everyone...
