AIUI, originally, A13 was to go to Fra Mauro, and A14 to Taurus-Littrow, but when A13 had its problems, the Fra Mauro site was allocated to A14, and Taurus-Littrow was bumped to A17; its original site, Marius Hills was dropped altogether.
A16 was originally Descartes Plains and it remained that way
A18 was to be Copernicus, but it was cancelled.
A19 was to be Hadley Rille, but when it was cancelled in 1970, the site was allocated to A15 (which was upgraded from an "H" mission to a "J" mission); its original site, Censorious Crater was dropped.
A20 was to have gone to Tycho (looking for a big black slab no doubt
)
I'm not sure how far through the planning the later missions were at the time they were cancelled, but since the cancellations were 2-3 years out, I wouldn't think much detail would have been decided on.
There is a little more here
https://en.wikipedia.org/wiki/List_of_Apollo_missions#Cancelled_missionsand an interesting discussion here
http://www.livescience.com/15900-apollo-canceled.htmlIMO, cancelling those last three Apollo missions was a very big mistake. Don Wilhelms, a retired USGS geologist who was part of a group vetting possible landing sites, said that cancelling the missions was
"a missed opportunity. You had existing technology at the peak of its effectiveness. So it was a waste."Harrison Schmitt was most outspoken about the issue. ...
"We never should have stopped building Saturn 5s and Apollo spacecraft. Everything that's happened since, including space stations, could have been done with that technology base. In addition, you would have continued to have the ability to reach out into deep space, a capability that included being able to divert asteroids in case one looked like it might be on a collision course with the Earth. For a brief, shining three or four years, we could do that with the Saturn 5."I am sure I agree with him, and I have little doubt that we would be a lot closer to putting astronauts on Mars if the Apollo and Saturn V programme had been allowed to continue.