You are right, I am using a sealed BB. For some reason I was thinking in my head that cartridge meant like a cartridge hub bearing, one you can replace. Yeah, my Shimano BB is sealed and all I can do it swap it out.

As for why I want to upgrade...the UN-72 is only slightly more expensive and it is lighter and a bit nicer. Though I have my eyes on a new bike, this is still my only true road machine and I like keeping it as nice/light/happy as possible.

I just have to find one...