Nosferatu,
Netscape 6.x is basically a stale old Mozilla build + commercial AOL crap.
Mozilla 0.9.8 is easily months newer (and better) than NS 6.2. One must-have feature that hasn't gotten into Netscape 6.x yet is tabbed browsing; not everyone needs it, but the ones that do couldn't live without it (at least I wouldn't want to).
But since NS is based on Mozilla, the GUI is basically the same. When you install Mozilla, I believe it picks up all your existing NS plugins. Both *should* coexist on your system at the same time. I don't believe they can seamlessly share the same profile though.
I think in short, the answer to your question is "not hard at all".