additional info should be used for non upgrade requests, and then most hotels repect and act on it. field is not there to say i want my HH upgrade.

If hotel is going to upgrade you it will happen whether you ask for upgrade or don't ask in additional info. If not upgraded at checkin time one can always ask politely if any chance of a nice room or upgrade. I am not too bothered on 1night stays but will ask at checkin on my multinight stays,

As an elite when booking twin bed room type I often add comments, as usually hotels EXEC Rrooms predominantly/only have King Beds (no twins), which is often the expected/hoped for upgrade.

Ocasionally I don't want an upgrade from twin room to an Exec King, or conversely only twin bed type available and I'd like a King Bed.

I have never been to date changed from a King Bed room to Twin beds room type. Booking King Bed I don't leave any additional info comment.

eg additional info .....

"I booked+need twin beds. Please do not change/upgrade to a king/queen bedded room"

"King Bedded room preferred please. (only twin type was available to book)"

note
I still get my upgrades where hotels want to upgrade elites, and conversely also believe I get more/better upgrades as the backroom person allocating rooms/upgrades appreciates no mention of "I am HHD can I be upgraded" demand no matter how politely phrased.