Hi all,
Installed win 10 (when it was free) from win 7. Didn't like it at the time.
Formatted the drive before I put back win 7 
I would like to go back to win 10 now, but is Microsoft
going to charge me for a new win 10 activation key?
 
Thanks! 
 

You should be able to skip entering the cdkey, if you already entered a windows 10 cdkey on that machine.

aka_STEVE_b
yep, you should be able to just install win 10 again using their creation tool on a usb drive.
 
https://www.microsoft.com/en-us/software-download/windows10
 
it should automatically recognize the hardware ID's from before and authenticate it later just fine.  I have done it many times on many systems already.  You can skip the cd key entering part & the registering part .
 
 
 

Using the tool to upgrade this PC to Windows 10 (click to show more or less information)
Here’s when to use these instructions:
You have a license to install Windows 10 and are upgrading this PC from Windows 7 or Windows 8.1.
You need to reinstall Windows 10 on a PC you’ve already successfully activated Windows 10.

If you are installing Windows 10 on a PC running Windows XP or Windows Vista, or if you need to create installation media to install Windows 10 on a different PC, see Using the tool to create installation media (USB flash drive, DVD, or ISO file) to install Windows 10 on a different PC section below.

Note: Before you install Windows 10, check to make sure your PC meets the system requirements for Windows 10. We also recommend going to the PC manufacturer's website for any additional info about updated drivers and hardware compatibility.

Select Download tool, and select Run. You need to be an administrator to run this tool.
On the License terms page, if you accept the license terms, select Accept.

On the What do you want to do? page, select Upgrade this PC now, and then select Next.
The tool will start downloading and then installing Windows 10. 

 

+1