Are you serious? First off, I've built and upgrade a lot of PCs and workstation for friends and relatives. I'm neither a certified tech nor working for a tech company.

When it comes to access of the client OS, there should be a confidentiality paper signed during the drop-off. They demand access not to go through your stuff but to check if the hardware installed are working properly through some stress test. Don't forget that hardwares require driver installation or update. If you want the best service, then maybe let them have access to your OS? Unless you dont have some shaddy stuff you're hiding, you shouldn't be worrying about them being exposed.

You said you could have done the upgrade yourself, why not do it if you don't want strangers accessing your PC? Clients like you are the worst when they denied access to the OS and cries about how their PCs aren't working properly or fails after a few weeks.