As mine was a reproduction board the difference was baked in so not a trivial change perhaps - U25 is is an extra LS138 instead of one of the RAM chips and U26 is a LS08 instead of another and U16-U19 are then rewired.
Here is the relevant bit of the diagram I was supplied with the board....
for those following along the original circuit is on the bottom left of Page 30 of the ETI article.
