On the other hand, if you keep your 8-speed shifters and cassette and crankset, you can upgrade the derailleurs. Derailleurs don't care how many gears there are, the shifters determine that. Just make sure the shifters can handle the range of cogs you have. If you wanted to upgrade the shifters to 105, you'd have to look for an old 105 shifter from the days when 105 used 8-speed (guess it's now 10-speed) or convert to 9 or 10 speed.