If you replace a bad motherboard, you will need to reactivate. If the replacement is identical, you will have no problem.
If the replacement differs by enough, there may be an issue.
If the online activation fails, you will be given a toll free number for a second try at unaided activation.
If that also fails, you will be given a number to talk to ms.
MS is primarily interested in stopping the spread of piracy and making certain that the particular copy is genuine, and is being used on only a single pc(read motherboard)
The key answer is to assure ms that you are using the copy only on the pc you are trying to activate.