You will be best off if you connect your drives to the new MB, making sure that you connect the system drive correctly so the new MB will present it as the first drive, and then immediately do a server reinstallation. It's possible that you could boot your server; more likely it would blue-screen, though.
Other precautions I would take: set all SATA ports to "IDE", "Legacy IDE" etc. mode. In theory you will be giving up a small amount of performance, but in reality there are other bottlenecks that will overshadow the losses from this. Setting your SATA ports this way will allow WHS to use built-in drivers in the installation phases, and will make for a much easier reinstallaiton experience.
See
this FAQ post for more information about how to perform a server reinstallation. (Don't let the title throw you. :) )
I'm not on the WHS team, I just post a lot. :)
