As a long term Gold Member (2+ years now), the experiences I got upgrading will vary. Depends from hotel to hotel
Stayed multiple times in some hotels:
Some is 100% upgrade chance to executive room ( Doubletree JB, Hilton KL) even if I don't get it in the app, asking at check in helps...
Some is 50% upgrade chance to executive room (from base)
Some is 10% upgrade chance even after asking. Normal reply, hotel is full etc...even though I stay there like 10 times a year
Some policy is one step higher but no difference unless there is a fire alarm and more stairs to go down. Don't laugh, happened to me once...
Conrad (cent Singapore) 100% so far upgrade to the next higher room policy
Booked 2 connecting rooms no way they will upgrade .
Booked family rooms no way they will upgrade
HGI even though not their policy to upgrade sometimes there is a 25% chance of upgrade to a "better view"
Thankfully those days as Gold are nearly over...