I was on the same situation, I called their tech support, and I was told that I need to pay ~500 dollars if I want to upgrade to 8.x
So, i installed 7.6 on the new server, upgraded to 7.6.1, did all the patches /updates..Then asked them for temp license, restored the backup from the old one..Which did not do the restoration properly, Mysql databases for a couple of domains are empty (they were created, but no content from the old server), i had folders with custom permissions - they were not kept on the new server..
Migration tools did not work, I got errors saying that the versions of the agent / manager are different (i downloaded exactly what was on their utilities page for Plesk Windows 7.6).
Overall, it was an experience a little bit painful..