Others have said this before but it chimes with my experience.
Don’t check in online the day before (many hotels do things like this overnight the night before).
On the morning of arrival take a look at what is available, if there is an upgrade consider taking it and checking in but don’t download a digital key.
Stop by the desk for a key and politely ask for an upgrade.
On not being able to select rooms at all, although I have seen that on my stays this year I have had a choice on most occasions.
Since restrictions eased a little here in the U.K. I am 4/4 on upgrades, but 0/4 on upgrade to suites.