how to directly use existed vm after installing new version?

[lanblack]

Dear Sir:
My PVE currently is 6.4. I don't subscribe and want to install 7.4. I just tried to install 7.4 on another lab server.
I can normally use after restoring the back up vm from 6.4. If I want directly use 6.4 vm, how can do that?
because i the each one vm is lvm disk.