Inkundernails says "The other times I got upgrades beyond the standard upgrade."

As a Diamond, you will almost certainly get a "standard upgrade" unless the property is booked out. Some properties are more generous than others, so it may be Executive floor, or indeed a Suite. If you wanted to make sure it is a really nice upgrade, then follow Inkundernails' advice.

Just as the standard upgrade policy, the quality of the lounge offerings differ dramatically, particularly between the US and Europe/Asia (the latter have far superior quality lounges).