Apollo 16 far ultraviolet camera photos were scanned to digital in 1973 (or something, early 70's) and while they are hard to read, and how they were exactly decoded has been lost to time as the person who wrote the digitize program has passed... they can still be read. So NASA was still able to get them of 70's data tapes.
Lost all pre-apollo landing surveys? Do you mean Lunar orbiter? the Lunar Orbiter tapes from 1976 to 1977 have been read successfully some 8 years back. But I think their encoding was analog?
And yes, it took years of searching and a few tens of thousands of dollars to do it... but were successful. Not even NASA expected it would work, but people made it happen.
If there is a will (and money) there is a way.
Unless you talk about the unprocessed SSTV transmission from Apollo 11, those tapes have yet to be found and are presumed lost. Only the NTSC conversions are known to exist.
I have read, used and archived data from hard drives from the 80's, in anno 2017.