Here's a crazy idea. Let me know if it wouldn't work:
Clone the hard drive on your work PC onto a second hard drive or SSD.
Put the second hard drive or SSD it. Upgrade it to Windows 10 to claim your "free" upgrade.
Put the old hard drive in it, and enjoy Windows 7 for a few more weeks.
When they give you the old PC to take home, put your new hard drive in it and enjoy Windows 10. Then reformat the old drive and use it for additional storage.
I did something similar, as I am not ready to put my desktop to 10 quite yet.
- Cloned my SSD to a hard drive when 10 was 'ready'.
- Upgraded the hard drive to 10.
- Put the hard drive in a drawer.
- Just last week, downloaded the latest 10 to a flash driver.
- Pulled the hard drive out of the shelf and did a fresh install (formatted the drive first) of the latest build off the flash drive.
When it completed, instant activation.