Problem with understanding EPROMs and how they're written to
EPROM cells can only be electrically written 1-&gt;0 and can only be erased with UV 0-&gt;1
So you can electrically overwrite as many times as you like, and get the correct result,  provided you never want to change a 0 to a 1.
To change even one cell from 0-1 you have to erase the entire array (well, unless you're really accurate with a UV laser and/or masking the bare die, but that's more like hacking).