Later versions should open older versions especially when they are as close as the two you mentioned. If that were not true probably 99% of the debut owners of Harmony 17 would have a problem with all the work they created.
Contact support directly about the trials.
When you buy a permanent license you start with the most recent and retain the links of any subsequent versions until you no longer renew your license. I have never subscribed but I recall reading that you get the most recent version at the beginning then if there are any updaters released you are given a link to make the switch yourself. I do not know whether you have an option to revert back. I think someone was able to do that with special assistance from Support though. The only way to get an older version that existed prior to purchasing a permanent license is by purchasing the Gold Support plan.