If you switch from a low durability bow(lets name it bow 1) to a full durability bow(lets name it bow 2),while aiming an arrow, the bow 2's durability will switch from full to the durability of bow 1 and if switch from bow 2 to 1, vice versa.
1. Open any world
2. Get two bows
3. Manage to get one of the bows quite damaged
4. Aim an arrow with the bow with full durability and don't let go
5. Switch to the damaged bow
This is currently only tested on windows 10 minecraft.