In general, I don't think the offers are random. I do believe they are based on a person's stay patterns for the previous year (or some similar period of time.) I was a Gold during the previous Megabonus and had only stayed about 30 or so nights and was offered the 25K offer. By the time this one came around, I was a Plat and have well over 100 nights this year, and I was offered the 35K promo.

As for how to ask, just be upfront. When I asked to change my last promo, I didn't tell the phone agent anything except that I wanted to change to the higher point offer. I was never asked how I knew there was one.