I have the same problem with my Burton Lexas. They fit my old Burton boots fine. I bought thirtytwos this season and now I have to jam the heel in, it's so tight that even when unstrapped it's hard to get the boot out of the binding. Like you, Im waiting til next season to get new bindings.
Never used the missions but you can swap them at home to see if your boot fits or not. Take both pairs to the mtn just in case.