1. Backup your Windows 7 install 2. Upgrade to Windows 10 (this associates your machine with a permanent license) 3. Restore your backup If/when time comes t

1. Backup your Windows 7 install 2. Upgrade to Windows 10 (this associates your machine with a permanent license) 3. Restore your backup If/when time comes to actually upgrade, you should be able to do so (or install from scratch) using the license you acquired in step 2.