I think dbpro is capable of better effects and stuff, but many don't use them (like shaders). My graphics are the same in dbc and dbp- just diffuse lighting, and your're still limited to the 8 lights. A lot of the stuff that dbp can do is dependent on having a fancy pants graphics card- like shadows and normal maps and things- which i don't have- and i don't want to get yet because other people don't either.
You do find you can shove in more objects and scenery and stuff before the game begins to chug though.
As for converting dbc to dbp projects, it's not that hard at all. I think the camera view command changed from degrees to radians or vice versa, and the rotations changed around abit, and you need to disable object zdepth for ghosted objects and a few more things, but i've had more trouble with version updates in the past.

You'll be able to click on this someday.