Get MS account and activate OneDrive on the old setup.
Move data to the mirrored folder (wait for them to sync to cloud)
Remove the old primary drive.
Install SSD + W10, activate the same MS account
Watch data download back.
Connect the old drive to a secondary channel (non-boot)
Something goes wrong? Re-plug the old HDD into the primary boot channel (or change their roles in BIOS) with the old OS still on it - voila, as if nothing ever happened. Disk upgrade + OS upgrade make a lot of sense if combined.