Posting here since the answer the phone agent gave me made zero sense.

I qualified for DM on Sunday but the miles haven't posted (should tonight) so my status still shows as PM.

I called in to see if if checked in for my next flight (tomorrow) at 24 hrs would leave me sat as a PM on the UG list, or if when the status updated it would kick me upward.

Agent then said it didn't matter because my status wouldn't change until the end of my trip. (Something I know is incorrect, because my PM status kicked in halfway through a trip a couple of months ago.)

So, back to the original question. Check in at T-24, or wait for the miles to post and check in then?