Sidegrade only, no reservations
The problem with the MSWin10 free version is you can't just reserve it, or simply move your license to MSW10. You have to actually *INSTALL* it in order for the sidegrade to be considered valid. So if you want to make sure you have the free version before end-of-July, you will have to:
1: clone your hard drive
2: switch to that HDD to make sure the cloned version works
3: image that version, just to be sure you have more than one copy of a working configuration
4: run the MSWin10 migration/sidegrade.
5: Make sure you have that license number stored someplace,
6: make sure MSW10 is running to some extent
7: install chocolatey, install classic-shell & a handful of other basic applications like Firefox (if not already there)
8: run some utility like SpyBot Anti-Beacon to remove/block telemetry. Shut down and un-pin as much of the default MS crap as possible
9: INSTALL LINUX MINT as a dual-boot option, so it's in place for when you can no longer stand the stench of MSWindows
10: pull that drive and store it in a hazmat bag
11: reinstall your original drive and carry on with what you had been doing before all this mess.