So.. Um
If I remember correctly, I use to be able to change sprites...
But is that true? And if it is can I change a sprite than I bought with the pod?
Changing sprites?
If you wanna buy a new sprite, you can find them on the Supply Depot which you access by pressing K. Note that thanks to the recent promo, there may be some reskinned sprites available for less than those in the Supply Depot on the AH or in the trade chat, but don't take my word for it.
When you remember being able to change sprites, maybe you are thinking of the early missions where you try out the three sprite types? That's all temporary. Once you choose your free sprite after those missions, you can't change sprites without buying a new one.
To be able to change sprites, you have to buy another one in the supply depot (you won't lose your current one, I think). You can also replay missions 2-2-1, 2-2-2, and 2-2-3 to try them out again, but they'll only last for that mission. Also remember that you have to go to the inventory and re-equip your battle sprite after replaying one of those.
Um. If you have multiple battle sprites (out of their pods + bound to you), you can click on one in your inventory and say 'equip', just like you can with everything else you own. If you have one battle sprite, you can click on it and say 'unequip' and then re-equip it, to change from not having a sprite to having one... just in case you want to test it out.