I was playing Black 2 and White 2 tonight and I found, to my surprise, you could actually just reset to change the swarms for the day (like any other daily event, like obtaining fossils and whatnot), at least provided the last save was from the previous day. I didn't know that was possible (mostly since forcing the swarms to cycle in Gen 4 required messing with the DS's clock and calendar). Has it been that way since B/W? I never noticed it then...