Difficult painting doesn't exist on good plastering - painters make their money by either being fast or by understanding prep...
You've already done the prep by plastering properly so it's just a case of whacking it on.
If the customer does happen to ask something you don't know the answer to then just be honest and tell them you're a plasterer.
You've already got your foot in the door after you've plastered and they're happy so in my view, it's downright r******d to walk away from work that you could make more profit on.