Better expect some invoices then
If my clients go to work and discover any machine has been upgrade without authority, they better pay my invoices for reverting the computer back and expect a computer misuse knock on the door. And no doubt the clients will be seeking damages for lost work.
I know for a fact that one of my clients with 60 desktops (workgrouped, no domain, multiple external users), they run software that fails to run under 10. Been there, tried it.
Biting the hand that feeds IT
