If you take a look inside the bottom of the flash where the battery pack goes you'll see 3 pins. One is common, one for AA 1.5x6=9v, and the third for niCd 1.2x6=7.2v. Hence the different packs.

If you can find a place that rebuilds battery packs they may be able to replace the niCds. NiMh cell have different charge characteristics so not really recommended unless you change the charger.