Downgrading to 3.4.0

This evening I tried to downgrade from 3.5.0 to 3.4.0 on a Full Boar 4 going out as a dry hire at the request of the client. I used the ISO and the installation appeared to run perfectly, but on reboot the Start Windows did not appear. In the end I upgraded to 3.5.1 and I hope there are no issues with the show. Is it possible to downgrade?