I think you would find it less troublesome to reinstall Windows after changing the motherboard as you may well get driver issues further down the line. I know that Windows 7 comes with loads of driver and no doubt will have most of them for your Mobo, but you will still find that it is necessary to use the driver disk that came with te board.
I myself will be replacing a motherboard myself this week after my current one developing a fault, although I will be replacing the board with a similar product I know from past experience that it is better,in the long run,to do a clean install. I must admit it is reasonably easy as I use a SSD for boot drive and have Windows 7 SP1 as an ISO so it is a relatively painless process.
I'm thinking of finally going 64Bit, I shied away from this as there seemed to be a lack of drivers & software a year or ago but as that's all changed & 64Bit drivers are available for this reasonably recent DELL desk PC.
My question is - Will my Win7 serial code which was for the 32 Bit version work with 64?