When a ship declares a barrel roll, it chooses the left or right side. If a starviper cannot complete a bank barrel roll to either the front or back, may it rotate the template around to still barrel roll to the same side but in the opposite front or back direction? The rules refrence does state that the ship may change its final position from front, middle, and back.

Consensus interpretation is that you select the bend (forward or backwards) along with the side, and are limited by the three final positions (front/middle/back). That is, you cannot change the bend of the template if it does not fit, and will instead fail the barrel roll.

Rule of thumb as I know and promote it - once you've laid the template down, it's down.

Thats why you get to "fix" final position of a regular barrel roll but not the direction of a boost.

You will only ever have 3 final positions on a barrel roll regardless of template orientation. The direction (including banked barrel rolls to front or back as these are directions) cannot be changed if all 3 positions cause the barrel roll to fail. The ship is returned to its original position if the barrel roll fails.