The Buffalo Bills made a change midway through the season at offensive coordinator by firing Ken Dorsey and replacing him with Joe Brady, who was the quarterback coach at the time. Brady certainly did some good things with the offense and should draw heavy consideration to be the full-time replacement for Dorsey.
However, after promoting Ken Dorsey into the role and it not working out well, the Bills should do an extensive search for his replacement. This should include candidates not outside the organization and would solidify the belief that Brady is the right coach to call the plays for Josh Allen and this offense.
There is plenty of options out there with varying level of expertises, including a number of former Buffalo Bills, that warrant at least an interview or being considered for the role.