No matter what CMS you use coppermine with or what bbs your bridge coppermine, you will lose all the users information you have so far (the pics they uploaded will remain, but their use accounts will be disabled). This is what integration/bridging is about: coppermine disables it's own user management in favour of the user management it get's bridged with. Using this design it is able to nicely integrate with all kinds of other apps, with the drawback of leaving all coppermine users during this process. If you have just started your coppermine page, you probably don't have that many users, so it shouldn't be too hard to re-create them in the other apps you decide to use.
The question what CMS to use exactly is merely a matter of taste and what you want to accomplish: phpnuke, postnuke and all those nuke clones bring you a close visual integration of coppermine into the overall site, and they are comparatively easy to administer and maintain. Phpnuke is very buggy and insecure though, and postnuke as well afaik. Our fellow devs of
cpgnuke have created a nuke port of their own that comes with coppermine integrated and with a more secure code base, but you have to understand that the coppermine versions cpgnuke is based on differs from "our" standalone version, so there's little I can tell you about compatibility etc. Another well-made port is coppermine for
PragmaMX, which mostly has a German user base. The same things apply however as I mentioned for cpgnuke. Our friend Cas has created a coppermine hack for postnuke (
pnCPG) that enables standalone coppermine as it is right now to run within an iframe of the postnuke site, which has the benfit of compatibility to "our" standalone coppermine, but the drawback all iframe solutions have.
Personally, I prefer "real" CMS like "
mambo server" (or
Typo3, which sadly doesn't have a coppermine extension yet): those systems give you more flexibility in terms of page design and can be bridged with the standalone version of coppermine, but you need to invest more time to get those "bigger" apps running in the first place.
HTH
Joachim