I think that as long as you reasonably set up why any deviations in their voices might happen (ex: injuries, PTSD, etc.) the reader will be willing to go with you wherever you want to lead. There's some fic out there in various fandoms that I dearly love and the characters in it have wandered pretty far from their on-screen/on-page characterization but I like it because they've given me enough background to understand how those changes might have occurred within the realm of possibility.

Also, yay more Andy/Eddie!!!