it's called 'renegotiation' - as in you negotiate all over again and sign a new contract. If they don't agree on a renegotiated deal, the current one stays in place.
I woudn't be confident in bringing in Glennon or any other QB that sat on the bench for 2 years. Flaws and all, our offense is pretty solid when healthy, and pretty decent when not. If you bring him in as a backup , ok, but I would spend all our energies on getting a strong coaching staff with a strong DC. As many have posted, this offense with an even marginally better defense has us in the playoffs.