I swapped my double for a compact double, and I did it myself. This was pretty straight-forward with my bike as I went from Shimano Dura-Ace double to Shimano compact double. It's all in one piece, the bottom bracket, the right crank and the 2 rings which are 50/34. When the crankset/BB is inserted through the hole in the bottom of the frame, the left crank is then bolted on the left side of the BB.

I didn't have to change any shifters, chain, or derailleurs. The FD was positioned a little too high for the compact, as the original double was sized 39/53, so this was shifted down a little bit - easy. I did have to spend some time fiddling/tuning the gearing into the FD as there was a little rubbing of the chain initially.

You should figure out what sort of cycling you'd like to be doing and there are charts/figures/sums you can do to work out what gearing ratios you'll get between one set-up to another. The range of gearing you get depends on what rear cassette you team the crankset with, for eg. 11/23, 12/25, 12/27. I've got the 12/25 and I've found that in comparison to what I originally had, which was the double 39/53 teamed with the 11/23 cassette, I've got equivalent of 3 lower gears for going up hills than I originally had, and have lost 1 gear at the top end. Seeing as I'd never used that one anyway, it was no problem. I use the larger front chain ring more often now. It takes a little getting used to but I'm happer with the changes.