I already started from scratch; seems to work so far.
It should be OK if the old 2016 installation isn't needed for the upgrade activation to 2018; I don't think it is needed, but will see.
Just sent for my activation code.

BTW,
Songs and Lessons take up 10.5GB and Soloists take up 13.5GB.
The rest is only 4GB of the 28GB install.
That is without Real Tracks and Drums, obviously.

Dan