I'm replacing the shimano sora group (triple) on my bike with 9 speed ultegra (double). I'm going to leave the sora shifters on for now as the ultegra's need new hoods plus I can't be bothered fiddling with the cables.
From what I understand I will need to leave the 8 speed cassette on the bike for the shifters to work with the 9spd components. Is this correct? Also I have a new 9 speed chain that I want to use, will this work with the 8sp cassette?
my understanding then, is you are going to change over to 9 sp except for the shifters and the cassette, right?
Why don't you leave it until you are ready to do the whole switch. The rd can be used but it's going to need set up which will need redoing when you go 9sp and the front can't be touched until you switch everything out (new cranks, right?). The chain will work but you will likely have to cut it to a different length for the 2x9 setup and you can't cut twice to make it longer!