I think the newer chargers are backwards compatible with older batteries, batteries of different voltages (so you could go pick up a newer 18v batt and be good to go again). Give me a few mins and I'll go check my 19.2V hammer drill'* manual [(first time I cracked that manual out too)[/size]
I've replaced the cells in many battery packs, but not in several years now. I used to get my cells from Multiplier but there are MANY suppliers. The hardest part is getting the battery case opened nicely. After that you just tacweld/solder in the new cells in place of the old ones.
I didn't know that you could replace the cells cheaply in rechargable batteries. Most of the cordless I have are either Makita, or Dewalt, and after about 5 years they don't hold a charge as well. 7 years is quite good for the life of a rechargable. The dewalts I use are 18V. and the makita'* are 9.6V. I get more life out of the 18V.