Well, you can bet if you free upgrade a retail windows 7 to windows 10, you aren't gonna be able to reactivate the windows 7 on another hardware. Unless I see an official statement from MS about the issue, I will assume the worst in this case. That said, most people will have OEM license and SLIC 2.1 in bios, and this will not be an issue.
I have a retail Technet key for 8.1. I have backed up the phone activation through ATM. I have been using images for 1. clean 8.1 install using this method. 2. 8.1 to 10 through upgrade. 3. clean 10 install. In none of the above three cases have I encountered activation problems when I restored images. I go back and forth every 2-3 days between 8.1 and 10. However, I do not know what will happen in the future. I have multiple 7 keys and also 4-5 keys for 8 and 8.1. I guess I will reserve 1-2 keys for situations when I do not need to go to 10. That is unlikely though. Although I find 8.1 more stable now, maybe 10 will get better over time and in another 3-6 months, I will give up on 8.1 completely.
ive managed to reactivatre my 8.1 when i downgraded after w10 upgrade so im not sure this is real maybe after a year will become defunct idk