It is not possible to enforce royalties on previously minted nft’s. There are no restrictions on how an asset is send if it resides in a “normal” well behaved address. That is, the current owner can send it without any logic other then his own.
However, these assets can be brought back into such a royalty system if the current owner agrees with that. What one can do tho is create a multi-sig address that needs two signatures to spend an utxo. The first one is that of an owner. The second one is that of a plutus script that enforces two things.
- In the transaction royalties are payed.
- The asset is sent to a similar multi-sig address that abides these rules.
But again, note that for previously minted nft’s to abide to this system they have to be sent to such a script address in the first place, and the current owner has to decide whether they want to do that. In addition to this owners have to have a service to index which nft’s they hold since normal wallets will not index such multi-sig addresses. There is a CIP about this [1]
What is also being done is that market places integrate these functions. That is, they make plutus scripts that enforce the royalties (see the plutus script of spacebudz as an example [2]). But an owner of a previously minted NFT can always go to another marketplace that does not have such a function to optimize the trade and maximize their profits.