SeventhCrow brings up some very good points.
In addition, you may want to try to understand New Relationship Energy, NRE. Is that feeling of euphoria yoy get when you start a new relationship with someone you really like. It is very natural and lasts about 6 months to two years. During that time it is very easy to be in a relationship. Everythink is a cute little quirk. Later on, that quirk becomes that annoying habit you wish they would just stop!
So this applies in two ways to you. One is that if you go poly and meet someone, you may feel NRE. Your fiance will heave to deal with the jealousy and worry that you could leave him for someone right now. That is something that can be worked through by careful communication.
The other issue is that you may be in NRE itself with your fiance. Seven years is a long time to go. NRE helps us set up relationships that last. I am not sure how much quality time you can get with him in jail.
I hope it works out for you.