The battery is part of the backcover. So if you want to replace the battery, you have to replace the backcover.
Regarding the memory effect, even if Ni-MH batteries would have been used, you wouldn't have to worry. For the BeoCom 6000 Ni-MH batteries are used, I have several of them and it is very rare that a battery is broken, and that is something I hear very often from BeoCom 6000 users. Bang & Olufsen have used a very good charging system for the phones which ensures a very long lifetime or the batteries!