Buyer asked for commercial rights after the gig is done

[segakucing]

Hello, I wonder if there is an “official” way to grant the commercial rights to the buyer after the gig is done.
 

Some buyers ordered me illustration and some months later, they decided to ask for commercial rights. I know the procedure if they need the commercial rights before the gig start, but I am not really sure with this case. Usually, I just made a new gig and send the related illustration to them, stating that I (as the seller/artist) grant the commercial rights of the picture (order #xxxxxxxxx) to (buyer’s name).

Is it enough or is there any other way?

Thank you.