I have the Enterprise version of 8, but am no longer with the Enterprise. If I buy 10 and install it, will it break my existing VM and Windows?
My Mac is running Yosemite 10.10.2 and Parallels 8 is working OK for now.
Your Virtual Machine will remain the same even if Parallels Desktop version is changed/upgraded.
You can install version 10 and run the existing Virtual Machine successfully.
Parallels Desktop 8 isn't supported on Mac OS X 10.10 and you may lose access to your virtual machine with any OS update.