When did the backhand change?

I am assuming you mean 'when did it change from single-handed to 2-handed'?

I think it mainly changed in the earlier days of Jimmy Connors and his girlfriend of the time, Chris Evert, who were both virtually top-ranked in the world. Both of them were using two-handed backhands.

Then, not long later, Bjorn Borg and Ivan Lendl had long-term great success using top-spun backhands - which had been somewhat of a rarity up until that point. With the advent of more European clay courts, the top-spin backhand established its place.

So, in effect, as grass became a less favourable surface in the world, top-spun backhands have become more 'the rage', and much more use of the two-hander as well - even for many forehands.