Yes, at the time, I was upgrading from Win 8.1 to the first Win 10. Not only did I make two system images first onto external media with two different apps, but I also made a clone out of extreme caution. Then, I thought, why not have both Win 8.1 & Win 10 attached? He not only said it was doubtfully legal - but that it wouldn't work. I can see now he might have been thinking of the BCD.
Anyway, yes, by all means, have a shop detach the parent of the clone. On second thought, don't wipe it until you know the clone on the SSD is really working.
OK. Good luck, & keep us informed.
Edit: It's possible that BCD is bad enough that it still will prevent an upgrade even with the HDD detached now. Have the shop do both: detach the HDD first, & then regenerate the BCD. After that, try the upgrade.