Some users have recently had their accounts hijacked. It seems that the now defunct EVGA forums might have compromised your password there and seems many are using the same PW here. We would suggest you UPDATE YOUR PASSWORD and TURN ON 2FA for your account here to further secure it. None of the compromised accounts had 2FA turned on.
Once you have enabled 2FA, your account will be updated soon to show a badge, letting other members know that you use 2FA to protect your account. This should be beneficial for everyone that uses FSFT.
Im faced with the same thing. Im going from a XP 2400 to a 2500 Barton. i know I dont have to rebuild but it's a windows box that has the same install from 8 months ago. Its amazing the speed increases when you install a fresh MS OS. So for me I will but for you, if its running good now no need to redo it.
If you haven't been keeping your system running clean and efficiently, now may be a good time to consider reinstalling the OS. Otherwise, if you're just swapping CPUs you probably won't have any problems.
Like the guy above me said, if you haven't formatted in a long time, it may be worth the hassle so you reap the full benefit of your new CPU. If everything has been doing well, you shouldn't need a reformat.