A family member loves their 10-year-old Samsung S5 and has a few new ones they bought on eBay. Yeah, it’s old, but they are comfortable with it and prefer not to change it if possible. The problem is they won’t update even to their last software update from a few years ago. Is there any way of working around that? Would cloning it to the phone they currently use help with that?

No

Not through a provider.

Jailbreak/root, if you can find the update package to install:

https://www.wikihow.com/Root-a-Samsung-Galaxy-S5

Unfortunately, if you mess the procedure up, you're likely to have a "bricked" phone.

Thank you both for the help. That takes care of that.