While code provisions seem to prohibit the use of 20 amp receptacles on circuits of lesser ampacity, It seems entirely harmless to me so long as the circuit is properly protected based on the conductor size.
So if you are confident in the facts as you presented them, I would just leave them alone-- as I would if it was my house.

And when the house is sold and the new owner is not so knowledgeable?

For the little cost and effort involved, I'd change the receptacles and move with a clear conscience.