I have Cubase 6.5 installed on my old PC. I want to use Cubase 11 on my new PC. If I buy the update or upgrade, will 6.5 still work on my old PC, so I can occasionally work on old projects that require 32 but plugins, or do I have to update that to 11 as well?
It should work. You might need to update the eLicenser software on the old PC to recognize your 11 license. Your 11 license will allow you to run previous versions. Of course you would need to swap the dongle from one machine to the other as needed. Many people keep older versions installed for compatibility reasons such as you mention.