I am not an experienced water cooling enthusiast, so a question of difficulty will be subjective. I didn't think it was very easy because I was afraid of getting coolant on my sensitive electronic parts. Still, I've done it 3 times now and I only had a tiny spill one time and it didn't fall on anything sensitive.
Here is what I do:
I went to the store (Target?) and purchased a "Flavor Injector", which is just a plastic syringe that comes with a pair of metal needle tips. I say needle but the diameter of the tip is actually something like 2.5 mm. The tip wasn't suited to my purpose so I snipped maybe the last cm off of it using the snippy part of a pair of pliers. So now I've got a syringe that can get into a tight space and suck the water out. I also purchased some plastic containers to hold coolant.
Because the old pump was poorly designed with the reservoir cap on the SIDE of the reservoir (****), I now had to tilt my Hadron case to the side maybe 60 degrees from vertical. I propped it up with some books to make sure it didn't slip during this delicate procedure. I also had to tilt it so that the open panel of the case of facing up towards me, which meant that if I had a spill it would go right onto the motherboard. Sadly, this was my only option because of the placement of the reservoir cap.
I opened up the cap, and I used the syringe to drain as much of the coolant as I could get to. Then I squirted the coolant into one of the plastic bins I purchased. Following this I closed up the cap, picked up my entire Hadron case and tilted the machine around until some of the coolant was back in the reservoir. I repeated this several times; this took a while. I didn't get ALL of the coolant out this way, but I was able to clear most of the lines, allowing me to transplant the pump safely without getting any coolant on conductive surfaces.
I'm going to have to do this one more time because of the metal anti-kink coil inside 2 of the tubes. I put them in there because the instructions that came with the Hadron case said I should. However, the poster above you in this thread has really put the fear of me of corrosion for my water blocks, especially the delicate one on the GPU (GTX 780 Hydro Copper). So I'm going to have to drain the coolant, pull out the metal coils, cut more tubing, replace the tubing, put the plastic anti-kink coils I purchased on the new tubes, hook up everything, put the coolant back in, and then leak test again. Thankfully, draining the coolant should be A LOT easier and less risky because the reservoir of the pump I purchased (images earlier in thread) allows me to fill / drain the reservoir from the top.
If someone had told me how much work this would end up being before I started, I'm not sure I would have tried to water cool my system. I think, knowing what I know now, I would have custom purchased all the parts of my loop instead of buying the kit. That would have allowed me to make sure everything was compatible, put a drain fitting on the lowest tube, and really made maintenance a lot easier.