Do what I have done from personal experience with my pre-order.
And just leave it in your Pearl inventory.
It will NOT expire as I still have some of my pre-order items still stored in my Pearl inventory since Xbox launch in March.
I activate them as and when needed.
Edit: Sorry my mistake and you are correct and you can change the pre-order horse gear from horse to horse.
So thank you for that as even I couldn't figure it out until it was confirmed that you can, once again thank you for the help.