Had a very similar question closed on this topic. Didn't search for "rotation" when I was seeking dupes so I didn't see this.
The comment I'd add is that the belt should be able to extend along the new vector. Cities:Skylines does something similar with roads. You hold shift, and as you move the cursor it paths along the grid lines using one turn. I'm definitely a layman, but it seems like the pathfinding would be relatively basic compared to what is already in the game: extend lines from each pole (original and rotated) until they intersect, then follow that path. If the interior angle is < 90, give an invalid shape error.
Essentially I am trying to avoid having to place a rotated pole offset by two grid spaces, connect the belts to make the 90, then placing a second pole a very short distance away to make the next turn/maneuver. It would save a lot of clicks/clutter to just place the second pole and let the corner be unsupported. (If allowing the unsupported corner is a concern, there could be some limit to the offset, i.e., if [Lmax > leg(A) + leg(B) = 1.0L > Lmin] then either [leg(A) < 0.33L < 0.25Lmax] or [leg(B) < 0.33L < 0.25Lmax], where Lmin is some minimum length value used in place of the actual length to prevent short belts from having absurdly small offset limits.)
It isn't a huge deal, but all those clicks add up if you're snaking parallel stacks of belts around your factory. :)