1. Before we get into what I want to go over with you...tell me...what motherboard have you chosen for the AMD Threadripper 1950x CPU? Or have you not decided?
2. And if you don't mind the question...have you actually purchased that AMD processor? Or are you just considering it at this point?
3. I hope that by & by after you've built & worked with your new AMD Threadripper system you post a review of your experience with the new system.
3. Now...re your post query...
Assuming that your "old" HDD contains a viable OS, i.e., it's non-defective, bootable, and functions problem-free, it's conceivable that you can install that disk in your new build and it will boot & function just fine in the new system. BTW, I can't imagine this will be anything but a temporary situation that one way or another you plan to utilize the HDD as the boot drive for that new system. I can't imagine not having a SSD as the permanent boot drive. Could you comment on this?
Please understand that a successful transfer along the lines you contemplate is something of a crapshoot; obviously there are no guarantees here that it will work. But it may be worth a shot.
As a general proposition we have found the chances of a successful transfer increase when...
1. Both the old & new PCs are non-OEM machines, and,
2. Both PCs are either Intel-based or AMD-based systems.
Understand the above are not "hard & fast" rules. We've successfully transferred systems in spite of the above "criteria" not being met, but our "success rate" dramatically increases when the above criteria are met. I take it 1. above is met but obviously 2. is not. So I would not be unduly optimistic that you could effect a viable transfer.
So if by undertaking this transfer you're fortunate enough to gain a bootable, functional system as a result, you will, of course have all your programs, personal data, etc. available without going through that rather onerous task of installing a new system.
Obviously you may have to install different drivers of one type or another in the new system but of course these will be available from your new motherboard's driver installation CD/DVD. And you may be pleasantly surprised at Windows (Microsoft's) ability to pick up & install whatever drivers are necessary for your new installation without user intervention (especially since the OS is Win 10); although given the newness of the AMD Threadripper CPU and associated system I would think you probably will be dependent upon the m/b's driver installation CD/DVD.
IF YOU DO DECIDE TO UNDERTAKE THIS TRANSFER ATTEMPT WE STRONGLY SUGGEST THAT BEFORE DOING SO YOU CLONE THE CONTENTS OF THE DRIVE TO BE TRANSFERRED. SHOULD THE TRANSFER OPERATION FAIL (NOT AN UNUSUAL EVENT ESPECIALLY IN THIS SITUATION), THE NEW SYSTEM MIGHT - REPEAT MIGHT - MODIFY THE DRIVE'S OS IN SUCH A FASHION THAT THE OLD DRIVE'S OS COULD BE ADVERSELY AFFECTED, RESULTING IN AN UNBOOTABLE OS IF & WHEN THE DRIVE IS RE:INSTALLED BACK IN THE OLD PC. WE HAVE EXPERIENCED THIS SITUATION IN RARE CASES. HOWEVER, THE DRIVE'S NON-OS DATA SHOULD NOT BE AFFECTED. AT LEAST WE'VE NEVER RUN INTO THAT SITUATION.
Of course if the above is not of consequence to you because you have no intention of returning the HDD to the "old" system, then you can just forgo the disk-cloning operation.
So, all things considered, you may want to give the transfer operation a shot since it seems there would be nothing to lose but a bit of your time and you can always fresh-install the Win 10 OS on the new system.
Hope you'll keep us informed.