Automatic Elytra is a client side mod that will equip and unequip their elytra for them.
The player simply has to have an elytra in their inventory and when they try to fly it will be equiped.
The chestplate that was swapped out with the elytra will be re-equiped when the player touches the ground or water.
Note: This mod may be considered a cheat to some people / servers. Use at your own risk, I am not responsible for anyone getting banned for using this mod.