What my husband and me usually do (for example) when we would go to Soarin he goes on with my oldest and I take the little one and go on the Living with the Land ride or leave and go to Finding Nemo ride. By the time I would come back they are already done with the ride and I go on with my...
I have done it before. You just pay the difference. You are also able to upgrade 3 days before your arrival date. I recently upgraded for my nephew on his latest trip and he had no problems.