Dumb work-around: I needed to create a separate drawing file for sheet 3 of my 3-sheet drawing (another work-around). The first drawing file with sheets 1 & 2 had a detail view "A". When I created another detail view on sheet 3, which was in a separate drawing file, Fusion 360 defaulted to calling it view "A" also. So, I created a dummy detail view I didn't need, which became view "A". Then, I created the detail view I actually needed, which became view "B". Finally, I deleted dummy detail view "A" on sheet 3, and view "B" retained its letter designator. Not pretty, but that's true with a lot of things Fusion 360, especially when it comes to drawings. (AWS welding symbols anyone?)