They may or, people may drop their reservations. The challenge is the Park Availability Calendar is not updated real-time so does not show actual availability at the time you're looking.
If you already have a park reserved, the only way to change to MK would be to cancel your reservation and try to get a MK park reservation but this is risky as you risk not getting either.
If it were me, the only way I would risk my current reservation is if the park I have a reservation for is Green on the calendar so pretty likely that I can grab it back if I find MK is not available.
New dates may be added but there is no guarantee this will happen.
Another option is, if you can wait until the day you want to go to the MK, you may find availability then.
Sorry, probably not the answers you want to hear but hopefully helpful information.