The rules say that the golden brown action spaces allow you to "exchange goods in your supply for more valuable ones." (Trading Actions, p.16 in the English rules).
Once you put tiles from your supply onto a board you cannot take them back (i.e. you cannot return them to your supply). So what's already on your homeboard (or any other board) is no longer in the supply and cannot be exchanged/"upgraded" anymore.
So what's already on your homeboard (or any other board) is no longer in the supply and cannot be exchanged/"upgraded" anymore.
With the exception of sheep and cattle in their stables (which of course count as being in your supply although the stables are technically on your homeboard).
In another thread I asked if Ore and Silver can be removed once place since they are not "goods tiles". Apparantly the German and English rules vary on this point. Any insight? Much appreciated, thanks for the great unboxing and playthrough video!
I agree with what Felix wrote in the other thread: You're not allowed to remove anything (including ore and silver), but the English translation is a bit vague at this point, due to using both terms "tiles" and "tokens".