Hi, I got same problem BUT WITH ONE DOMAIN. First you need to buy Linux Hosting package. When you do that go to your Linux settings clicking the button - Manage - from My products page. On the Account box you have option for primary domain change. But before that I think you need to remove your primary domain from Plesk. For the others domains I' am not sure but you have options to change host and IP in DNS Zones. To reach DNS Zones manage Domains settings in Domains box on My products page. If you got SSL, you need to re key it and install it from cPanel. But I got one problem that I can't resolve. My SSL works fine but I got message -
This solution is if your Windows Plesk is about to expire. For other you can do it but you need to pay I think 100 dollars to change hosting system.
I understand. I can tell you about data from phpmyadmin. You have options Import/Export. When you want to move some table from database on Plesk to another database on cPanel first you gonna export database from Plesk then you gonna import exported file of database to cPanel. And you should change database connection data in php file. For Word Press I guess you need to install it like you are done that on Plesk, for backups of system files of Word Press and plug ins I don't know. Maybe their storage is on the same folder like Word Press files. Maybe you should try to Google it. If you think that you are not experienced enough to do that than my guess is that better thing for you to do is to pay for secure migration.