OEM whether it's a manufacturer's one, or one you bought makes no difference to the licence agreement. It's not transferable to another pc, any major upgrade like the motherboard is considered a new pc under the licence agreement.
Having said that Microsoft can be very reasonable when replacing a failed motherboard.
You will need to phone, you will probably end up talking to a human as the automated activation is almost certain to fail.
You are unlikely to need to reformat and install again unless you want to.
Just try it with your current instalation, it should boot. Once you're into Windows, just install the drivers for your new motherboard.
Now don't get too excited, you will still need to reactivate, you will have 3 days, it doesn't exactly make it obvious that it needs to be done, you will have to look for it in the System properties.