Memory effect basically has not existed for many years now. It used to be a serious problem, but today most NiCd failures are caused by people reverse-charging cells as they overdischarge packs in an an attempt to avoid nonexistent memory problems.
If you put a NiMH pack into the thing, you will get more capacity at first because the energy density of the pack is higher. But because the charger is not designed for NiMH cells, you will find that the battery pack doesn't last very long before it fails.
If the phone works, why screw with it?
--scott
"C'est un Nagra. C'est suisse, et tres, tres precis."