How will I get my Windows 10 Home authenticated again? I only have my
original Win 8.1 product key.

Clone the disk with imaging software...

Windows uses a "Disk identifier" parameter which can also be cloned and
keeps Windows happy (or directly edited). Of course, if the new disk is
larger then you have to expand the partitions after the clone. I did
that recently, replacing a 500 GB rotating rust with 1000 GB SSD (to add
dual boot) and Windows 10 did not complain.

I plan to replace the 500GB drive with the same size and type. I'm
going to first try using Macrium Reflect Free to create an image of
the entire HD