Wondering which is "harder" on koi (and bluegill) - a fall cleaning, or spring cleaning?

I can do either but need to know which one might ensure greater success w/ my fish. I have a 20K pond, 27 koi and who knows how many bluegill (I already recognize some bluegill will be sacrificed).

My main goal is to ensure that the koi are ok. The company that will be doing it is excellent so at least I have that going for me :). And no, I'm not going to do it myself. It's HUGE. lol

would appreciate any feedback and thanks!