Two points seem most important to me:
1. Any change to the predictability of our offense will be a good change. Defenses not knowing run vs. pass and not knowing the snap count will help a lot. Flexibility with things like screens and multiple formations and receivers changing spots will help. So, there is a very real and substantial upside to a new OC.
2. Tannehill is still a bit inexperienced and he hasn't yet mastered THIS system, and I'm sure learning a new system will take a while. He's very bright and a hard worker, so he'll get there, but the early season isn't likely to be smooth and consistently pretty if the system is a major change. He's still getting used to Wallace, now their timing may be more off.
There are a lot of innovative OC candidates out there these days, and if we can get one to take the hot-seat job, it could work out very well. But, most will want to wait for a more stable coaching environment.
Talent-wise, fixing the OL should do wonders for the running game and Tannehill's excellent play-action off of that.