The thing is, even the worst buyers know that what they are asking for here is unreasonable. It would not happen in any offline business that an order would be accepted as is and then weeks later changes would be requested not for additional cash but by a threat of cancellation. Even an idiot knows that is unreasonable.
When they know they are being unreasonable, it is much more likely that they will compromise somewhere.
I suggest that you are careful with how you proceed though, if you appear to respond to threats well then these may never stop. I would suggest that you say something along the lines of
“This modification request after the order has already been completed will take X amount of time which should cost $Y. If you are sure this is how you want it and there won’t be any further changes, I will make an exception for you and do this at no charge as I value your custom.”
You are being firm but fair and ensuring that if they do come back again that there will be additional costs.
As for the bad review/cancellation situation, once you have over a hundred reviews, one bad review should not really hurt. With the new level system, a cancellation could potentially hurt you more - and you would not be paid too! Like Mike, I believe in getting paid and I don’t worry about negative reviews - I have a couple and they have not affected me in any way that I can see.