Windows 7: Already have Windows 7 OEM, getting new motherboard

My PC is using an OEM copy of Windows 7 from the motherboard and HDD that came with a prebuilt HP PC. I'm getting a new motherboard next pay, and another copy of Windows 7. If I replace my motherboard and use the same HDD that has Windows 7 installed (OEM edition), is there an easy way to activate it with a code or something that will come with a new OEM edition? Or will I have to transfer all of my files over to another device, do a clean install, then retransfer?

Two things may happen, it will work fine (not common) or you wil have 3 days to activate (more common).
Go through the steps presented to re-activate your OEM copy. It will take a phone call to MS via robo-phone, that is a recording.

You should be presented with a long series of numbers, they will read back a long series to enter and you're good to go. This process will re-register the OEM Windows to the new hardware.

You could activate with the key on the new copy of Windows I think too, I haven't tried it with an OEM version, but you'd wastes a good key which you could use for another PC. The OEM and your purchased copy will have to be the same version, Home, Ultimate, etc.

If you want to keep the same Windows 7 installation then SysPrep to move HD to another computer to remove all drivers and activation so it starts up like on a new computer, then activate with the new Windows 7 license you're planning to buy.

In some cases MS will make exceptions for mobo changes on OEM version so you might want to first try activating with the HP Product Key on COA sticker, then see if MS phone activation will make an exception for the mobo change as increasingly reported: How To Activate Windows 7 by Phone Step-by-Step Guide

If not then buy the same version and install the Product Key at Computer>Properties. Or you can Clean Reinstall following these same steps as for factory OEM.