Sure, they can upgrade at any time, and it's not uncommon to see an upgrade either morning of or several days prior to arrival. But there's no obligation to upgrade until physical arrival (nor is any prior comp upgrade shown in the app binding until they've actually handed you the keys).
Originally Posted by
flyerfirsts
That said, there is one hotel I know that routinely does them first thing in the morning. I would give them a shout out, but fear that may lead to some overlord making them downgrade that procedure.
Many properties assign upgrades when they make the day's room assignments.