I have an old work laptop they said I can keep.  Can I buy a replacement hard drive with a new OS and just swap the HDs out?  HP laptop.  Thanks.

Maybe.

There may be other hardware that is not compatible.  Best bet is to try to run Microsoft's compatibility check on the old laptop (assuming it's running Win 7 or Win 8.)

1. Download the Windows 10 Media Creation Tool, then launch the program.  If you have Windows 7 or 8, the upgrade is free.  (There are some provisos... If you have the Home version of 7 or 8, you will only be able to upgrade to the Home version of 10.)
2. On the License terms page, select Accept.
3. On What do you want to do? page, select Upgrade this PC now.
4. Select Next.

Using a Windows 10 compatibility checker is fairly easy and self-explanatory. Microsoft has one, and it is known as the Update Assistant or the Media Creation Tool.

Not only will it scan your PC to tell you if it will run Windows 10, but it can also install it as well.

This tool is extremely easy to acquire and use, as you may see from the steps below:

Keep in mind that this is the same set of steps that you need to take when upgrading your PC from Windows 7 to Windows 10. The only difference here is that you don’t have to go all the way with the process.

If the program detects that your PC is too old to run Windows 10, it will send you a notification, and stop right there.

If you don’t get any notification, it means that the system is compatible and that you can upgrade to Windows 10 without worries.

Note: Another thing you should remember is that if you want to use the Media Creation tool to upgrade to Windows 10 and you do not have a valid Windows 7 or Windows 8 license key, you will need to also have a Windows 10 license key at your disposal, in order to activate it.  

It's a Windows 10 Pro machine

I was planning on buying a new Windows OS for it.

Since it already had Windows 10 on it, yes, you should be able to put a new hard drive in and do a fresh install of Windows.  Heck, you MAY be able to install windows 10 pro without a new license, depending on how it was installed/registered by the company.  You can download the installer from Microsoft and put it on a bootable USB or burn it to a CD/DVD if the laptop has one.

Mike

What benefit would there be to installing a new Windows 10 OS?
If the current one is unlicensed you can just buy a license and activate it.

If you just want to replace the system drive you should clone it with Macrium Reflect, then replace the existing drive.
Keep in mind you can do a Windows Reset (before or after you clone it) which will take it back to a default install.

As it is currently a company machine, I expect that before they let him take it, the hard drive will be removed, not to reuse the hard drive but to ensure no company data, licensed programs, etc., are on it.

Mike