In FIFA 23, if you want to change a player's position then you’ll need the Position Modifier items, consumables.
You can either get the consumable in packs or buy it on the transfer market, where it is listed under "Consumables". Any retraining of position always requires a "position modifier". So without the item you can't constantly switch back and forth. However, you can only retrain the player within their alternate positions.
How to apply a position change?