I've got two road bikes, a Giant OCR-1 with the stock mostly Shimano 105 group on it, and a Trek 1.2 with the stock Sora and lower set of components. I'm thinking that it would be fun to buy the Ultegra groupset for the Giant and put its 105 group on the Trek, thus (I think) upgrading both bikes for around $600 to $700. Fun project for the dead of winter.

I have plenty of standard tools and a bike work stand and I work on the bikes all the time, but I'm not sure if there's more than meets the eye to this potential project. Specifically, I've never pulled a modern bottom bracket nor installed a brifter. Other than that, it looks pretty straightforward.

Assuming I buy the right tools, should the BBs on these bikes present a problem? If the BBs are a big deal, any reason not to just leave the current ones in there and keep the new one as a spare? And is there more to installing brifters than running the cables and setting up the derailers and brakes?

Just browsing Ebay, there are a number of sets for sale. Some are stripped off new bikes and resold, and others are NIB; some are used. If I bought a group without cables included, can I use the cables from the bikes, or would I have to buy new ones?