I have the latest Insider W10 on my 640XL and will be receiving a new Nokia 1520 today which obviously will have Windows 8.1, I want to use my new 1520 stock for a while just to make sure all is good with the phone itself before doing any changes.. Would it be unadvisable to restore from my current W10 backup that is if it even would(thinking it may not restore from a different/newer OS).