For any entity with an existing owner NBT tag, any edits to said ag will not update in singleplayer until the player leaves and reloads the world. When NBT data is collected using /data get, the game shows that the Owner NBT data has been changed, however the game does not update the behavior of the entity to match the data change until the singleplayer world the change was made in is reloaded. If the entity with the changed Owner NBT data is destroyed by the time the world is reloaded, no change is made.

If an entity is created with the Owner NBT data that matches the UUID of an example entity "entity A" but is changed with commands to match the UUID of "entity B", the following will occur (assuming the player plays on 1.17.1 and does not leave and reenter the game):
* If the entity in question is an arrow/spectral arrow/other harming projectile, entity A is given credit for any damage dealt instead of entity B

• • neutral mobs hit by the harming projectile will become aggrivated at entity A but not entity B
  • If entity A is a player that has not yet killed a hostile mob and the harming projectile kills a hostile mob, entity A will earn the advancement "Monster Hunter"
    ** entity A's score in scoreboards with the "damage dealt" criteria will update but not Entity B
• if the entity in question is an ender pearl, entity A will teleport to the location the ender pearl landed instead of entity B
• if the entity in question is a dropped item, only entity A can pick up the item while entity B is unable to pick up the item

everything in the list above is exclusive to 1.17-1.17.1 and does not occur in 1.16-1.16.5; any changes to the Owner tag by commands in 1.16-1.16.5 work as intended.