Definitely a suite upgrade is not an entitlement for Diamond members but I have asked nicely on a number of stays: "By any chance, is a suite upgrade during my stay" and have had positive results on more than one occasion. I certainly don't think it is rude to ask and staff never seem to take it as rude. They will either respond with "unfortunately, no" or "let me see what I can do". But certainly not something you are entitled to, just a bonus if you get one.