I have just changed my MB and CPU in a Dell computer with a DELL Win 7 OEM license. Absolutely no hassle at all. Everything booted just fine. Within a day, I got a pop-up saying that my Windows activation was not valid.
One of the options on the activation pop-up was to "enter a different Windows Product Key". I selected that option and entered my Win 7 OEM product key from the decal on the Dell. It went online, confirmed the product key, and activated Win 7 on the new MB. Took about 30 seconds. If you have a retail Windows, the product key is probably on Win packaging somewhere instead of on the OEM computer case.
This same thing may happen when you change to a new boot drive, although I think it does the online reactivation without having to enter the product key. No sweat. But, just to prevent possible problems, I would probably not swap the boot drive, the MB, and the CPU all at the same time. I'd either do the boot drive upgrade first, get the Win activation out of the way for that, and then do the MB/CPU upgrade. I don't know if that's necessary, but that's what I did. SSD first with the old MB. Then the MB.
Actually, I swapped MBs three times as I had boot issues after a couple days (long story) with the new MB and swapped BACK to the Dell MB/CPU until I sorted it out. So I was reactivating Win 7 several days in a row.
Microsoft Office will need reactivation the first time you run it. Same thing: automatic on-line reactivation. May or may not need enter a product key -- one time I did, one time I didn't.
Norton Anti-virus will ask for the product key.