Hi svhe
Hope you well. Happens a lot. I have had it many times. Technically it is an upgrade even though as you say as Diamond lounge access is given. Sometimes though room is same but does have extras such as slippers, bathropes etc
I don't really bother too much. With online checkin I prefer to pick room I want. It is better to get exec room as they tend to be top floors.
At check in you can question it and see if you can get suite etc