Yes, call the hotel. At RPR we saw (through the windows) Home and Simpson on Saturday. We called star services from our room and the next two character dinings were Scooby on Wednesday and Shrek the next Saturday. So sounds like they do a good job rotating them.
We went on the Scooby night, and unfortunately it was only Scooby. Shaggy must have had a belly ache from too many Scooby Snacks. Scooby was great and spent a few minutes at each table (took several pictures, drank my daughter's milk...), but not a whole lot of time like I've read about from other people's experiences. It *was* June, and it *WAS( busy (not Disney packed, but steady). But the real disappointing thing is that my daughter said Scooby wasn't REALLY drinking her milk because he couldn't drink unless he took his mask off.