Question about models' "swappability"

Hey everyone,

i tried to google this one in vain, as you all know we can create and save our character's models, what i don't know is:

If we want to switch from one to another does it cost a voucher each time?

Or does it work in a different manner?

Does it cost a vouncher upfront and then we can swap back and forth between them freely?

Thanks in advance to everyone