I wouldn't cancel (unless you need to), even if JoJo and Goliath are still there. We went back in December and my 2.5 year old had no idea who they were but she didn't care. She loved the Little Einsteins and had a great time singing and dancing to the Playhouse Disney music. We couldn't even remember Goliath's name, but he was great with my kids (so was JoJo for that matter). The buffet was actually pretty good.
Would I like to see different characters - sure. Both my kids love Handy Manny and were in total awe of him when we took pictures with him later in the day.
Have a great trip!