At work I had my supervisor retire and they had me fill in until they found her replacement. Eventually after a couple of months they decided to just keep me in the position. I said sure but I'm not doing it without a promotion which they eventually gave to me.
Jacoby's contract was originally for him to be a backup who comes off the bench occasionally as needed. If the Cardinals are now expecting him to begin the season as a starter, I fully understand why he would want to renegotiate that deal. The amount of stress and the physical beating you take as a starting NFL QB warrant a higher pay.