I would still say that there's a slight chance this exclusion would refer to the "bonus" miles one can get for the re-booking and not that actually all of those tickets (for example award tickets in May/June) can't be re-booked. They could have just been a bit too careless about the way they wrote that text (no * 's to point out what part exactly these exceptions refer to etc.). I don't have any such bookings currently, but someone who does could try a "dummy-rebooking" as of tomorrow for May-June flights or even today for July flights, just to double-check...

I probably won't be changing the tickets I do have, unless they release the S21 plan (and unless it's certain that flying this summer isn't a somewhat available or good option).