Silverdragons idea about jumpers is good.
To remove the old battery if it's the barrel type, with 3 soldering points, first desolder (From the overside of the motherboard) the end of the battery with one solder point, then bend the battery up a bit so you can desolder one of the remaining two. I did this on one of my 4000's yesterday :-)
No need to remove the motherboard, but you will need to get one of those soldering-lead(?) suckers. (Sorry, bad english) :-) to suck the holes empty, so you can fit the new battery, altough Silverdragon's idea was good. :-)
@Silverdragon: sorry, didn't remember you short-nick :-)