I also suspect they wanted a buffer between IllumiNations and HarmonioUS because jumping right from the former to the latter would not reflect favorably on HarmonioUS. I think they're hoping IllumiNations fades a bit from fans' memories, so that the reaction to HarmonioUS is "Well, that wasn't so bad," rather than the "IllumiNations was WAY better!" reaction many might have had if the switch happened quickly.
I still bet we'll hear a lot of that last reaction, anyway. It's almost certainly going to be mine!