Maybe you will need to reinstall Windows. Maybe not. My experience over the last 10 years has been that using and old Win install with new motherboard works about half the time. Your odds are better if you stick with the same brand and chipset family (like Gigabyte P35 to G'byte P45). Your odds are much lower if you are changing between Intel and AMD processors.
Backup your data, plan for the worst, and hope for the bset.