That is really hard to say. I say that because the base offensive design is McVay's. For the most part, Sean calls the plays. Coen helps him put the game plan together weekly. As such he will understand how this offense is designed and how McVay's concepts are implemented. During the game, he looks for defensive tendencies which help McVay with his play calls. An example of what I'm saying is that game-winning play in the Super Bowl. They had seen the Bengal's short-yardage tendencies all game long. They used that QB sneak not with realistic expectations of success. But it was to set up the very next game-winning play. That is exactly why Sean told Matt to send Cooper to the outside position. When I saw that man coverage on Kupp I knew that was where the pass was going no matter how it was played by the CB. In other words McVay's OCs function like Asshole Face's OC Pete Carmichael. O'Connell helped McVay keep track of those tendencies.
This is why Ram OCs are in demand as HC prospects. Teams want to import the Ram's style of offense and team culture. That is what Coen did at Kentucky. He changed the culture, and he changed how the team was coached. Each week his teams were shown both on offense and defense, video clips of the most recent Ram game. He used it to teach concepts and techniques.
Having been on the staff in 2018 through 2019 it means that McVay does not have to "break-in" a new OC. The base offense isn't going to change because of him. He will give Sean his input but make no mistake it will be McVay's scheme. I'm hoping he will point out the primary issue with the run attack, i.e. either go with no lead block and revert to basically strictly using an inside zone scheme when running between the tackles or an FB or HB if he is going to stay with that power scheme. But IMO that will be the level of impact Coen will have.
Coen's impact will be felt in small but perhaps important ways. But for those who expect it to change the offensive scheme, I don't see it that way. The Rams have had one OC after another yet the offense remains the same. That in itself is proof of what I'm saying.