That's not how that works. You're talking about a pre-snap audible to the route.
Sight adjustments are a post-snap adjustment, Modifications made to a receiver's route according to the defensive coverage. The adjustment is made by both QB & receiver based on what they both see. It's possible that Britt was supposed to sit soft in the zone and recognize when he can turn it into a deep post. Maybe Keenum saw early what Britt saw late. Either way, even if none of that is what happened, Britt dogged it on the route because he didn't think he was the primary. You can see that on the replay that I posted.