As far as kits I am assuming that you have the H20-220 Apex Ultra right? (MCP655 pump, Apogee GT CPU waterblock, etc...)
Yes, one of the most tedious aspects of altering any watercooling loop is draining it. Now, if you are careful (and depending on how you have yoru rad set up) you might can get away without having to COMPLETELY dismantle the loop (i.e removing the reservoir, rad, etc). BUt, you will have to make sure that the tubing in the loop is drained because you will have to alter the loop to add the GPU waterblock for your videocard.
It is up to you if you can do all this without removing ALL your hardware but, as this seems like you are new to watercooling, you might err on the side of safety here.