Help me complete my new build and recycle parts for another!

Got my new parts for a new gaming rig yesterday! Ryzen 5 2600, Asus Rog Strix B450 MB, Corsair Vengeance LPX 16GB 3000 RAM, and Cooler Master H500 case.

This will be my new build.

Now I need to figure out the easiest way to get this new system up and running with some of my old parts and cobble together pieces from my old rig and an old Dell for another system for my girls.

Typically I just build a complete new system but in this case I'll be pulling my Asus 1060 graphics card, PSU, an SSD, and a storage HHD from my old gaming rig for this new rig.

Then I'll taking the HHD and PSU from the girls old Dell POS hotpodge grandma's handmedown and adding and an Asus Radeon R9 280X Graphics Card to put into my old gaming rig to give them something better to play on.

Recycled build:

My question:
I currently have 2 SSD's in my old gaming rig. A 120GB with OS and main programs and a 250GB with games.

Will I be able remove the 120GB SSD with my OS from my old rig and plug it in to the new and it will recognize the new hardware? It's Windows 10 Pro 64bit. Or would it be better to just leave the SSD with my OS in the old system, get a Windows 10 ISO onto a thumb drive, and install it onto the the 250GB SSD and try to reactivate the key from the old Dell system in my new gaming?

I think if you change the motherboard you have to re-activate Windows. I expect it will probably boot if you just move the drive over and then it will run some driver updates. I am not sure how much cruft will be left behind from the old hardware like drivers or whatever.

I remember some of those Dell keys used to be vendor locked and could not be used on a non-vendor system. I don't know if that is still the case these days with Windows 10.

In talking with one of our systems support guys at work, they said the easiest thing to do would be to keep the 120GB SSD with Windows 10 in my old rig where I'm not changing out the MB and CPU. Then do a fresh Windows 10 install from a USB drive onto the 250GB SSD in my new gaming rig and try to use the windows key from the old Dell. He said if that doesn't work I can get a Windows 10 key from Amazon for $6.