For now, at least, they can't be changed once you've made them. (Which I think is
really stupid, but sadly I'm not a Dev, otherwise I could do something about it

)
Best bet is to save before you do anything. You can check the list when you select one of the trophy bases to see what you can make and what it needs, so it's not entirely hit and miss. But due to trophy facing, for example, I'd love to be able to move my Ice Wolf and swap it with the Spriggan I have, because it would look better. Sadly, I can't...
Haven't got a full list on hand, but you can check before you make. Just save first
