Swapping hard drives to a different computer

WIndows 10 should not cause problems.
Make sure the drive isn't BitLocker encrypted.
Office 365 may cause problems since the license is stored in the TPM on the mainboard.

The W7 keys in the BIOS were unique to each system, so the BIOS checksum will be different, Windows will detect some hardware differences and there will be some points added to the change score.  A motherboard swap is 6 points.  I would expect a Windows 7 installation to reauthorize automatically due to the BIOS key for W7.

Windows 10 I am not so sure about.  The 780 series had BIOS keys only for Windows 7, so what is on the drive is a W10 upgrade key.  You'll know if and when the system comes up with a demand to reauthorize; in that case it'll be necessary to do a key challenge authorization.  Or you can go directly to "I admit it; I changed the hardware."

https://www.windowscentral.com/how-re-activate-windows-10-after-hardware-change

Quora claims that as long as W10 is associated with a Microsoft account there is no problem.  However, it does not state whether this is for W10 OEM or W10 retail.

https://www.quora.com/Do-I-have-to-buy-Windows-10-every-time-I-upgrade-my-CPU-motherboard-or-primary-storage

Before you go down this road, use the existing OS on the desktop.
And run the upgrade to Windows 10.
That should reveal whether the Windows 10 entitlement attached.
Simpler and less risk to ....

Both Laptops were known Vendor?
I've found that Major Vendor's systems HP, Dell, win10 activates on upgrade.

The other condition, is the login used a microsoft login?

"Both Laptops were known Vendor?" - old: Asus, new: Samsung
No, the login was definitely not an MS account on the old nor on the new one. He never used one.

@Michael Pfister
No BitLocker and no Office 365.  TPM not enabled.  So there won't be any of those problems.

@Dr. Klahn
Been down the hardware difference with XP when I swapped motherboards for a relative.  Just had to make sure I swapped everything over from the dead board to the new one (and the new board was a similar model to the old one).  Will do the same with this one.

@arnold
There is no hard disk or memory in this machine.  Machine was supposed to not turn on.  I bought it like that.  But it does turn on and boot to BIOS when I put memory in it.  Was told it was running Windows 10 so I'm presuming it had Windows 7 installed at one stage.

@McKnife
Interesting.

I do have to find a bootable CD I have somewhere.  This tells me what keys are in the BIOS.  If it's got a Windows 7 key then I'll be making the attempt.

You do not need to boot a CD to find bios keys (which are definitely different from machine to machine, by the way).
Download and run
rweverything from within your running windows (run as administrator via right click)

if it ran windows 10 at any point, the license is on the MS servers. all you need to do is run the windows 10 installer. skip the prompt for the product code.
then once installed you will see whether it is activated.

i had a dell pc  - installed win10 fresh all ok (activated digital licence)
after several months no more activation : could not reach your server message...

Either the authorization went and locked in on a KMS type of setup, or something else was going on.
Did you resolve the issue? address the license, reinstall the OS from a newly downloaded ...

i did reinstall from a dell image to solve it

This machine had no hard disk or RAM when I bought it.  No serial number on outside of case.  Gave it 8 Gb of eBay RAM and a 75 Gb disk from an old Dell.

Installed Windows 10 Pro 1809 with no networking.  Windows is not activated.  Installed RWEverything from McKnife's link (I have used it before).  SLIC said 2.1 so that's Windows 7.  Hooked in a network cable.  Windows is activated.

So I've got another Windows 10 Pro machine.  So in about a week's time I'll attempt the swap over.

Well, I've done the swap.  No problems so far.  If it holds up for a week then I'll close this question.

keep in mind- mine was 6 months later to show up
so check what licence activation you need/have

>>  so check what license activation you need/have                                  

Activated with a digital license according to Windows.

Office 2007 has asked for and got an activation check.  That passed OK.  So it appears Office noted something different. And that is probably the motherboard.  All the rest of the hardware is from the old system that got transferred across.

i mean a licence from the oem or from windows

There's a Windows 7 OEM key embedded in the BIOS.  That gives the upgrade to Windows 10 and hence is the digital license.

Both machines have this key and both have run Windows 10 before.

So I have swapped the drives from one SFF to another DT machine and I'm just waiting to see if Windows sees a difference in the OEM keys and complains.  Slight hardware difference as this DT machine has an extra PCI slot.

"So it appears Office noted something different" - windows noted the same, but it tries to activate automatically with what it finds as keys, whereas office 2007 does not try that automatically.