Miles seem to post especially on Tuesday and Saturday overnight with batch processing - upgrade "stickers" probably do too (it has been a while, so I don't recall.) You could check online Sunday morning and buy if necessary (they require two hours to post, though you will not be able to see them online immediately.)

If you do buy a few in advance, it's not a terrible thing - January 26 is the last day you can buy "sticker" domestic upgrades at $25 online as they go to $30 after that date.