First, you have multiple activations for each order so, if you "just" changed the mainboard once, it's not possible you won't be able to reactivate, unless you changed it 5 times without ever deactivating first, because changing the mainboard will JUST consume ONE of the 6 activations you have for each order.
However, even if you forgot to deactivate for 5 times in a row, you CAN deactivate yourself, using the Self-help site:
https://qlm1.net/virtualisoftware/qlmcustomersite/- First use the "Get License Information" option to know the computer IDs that has been associated to your activation key
- Then use the "Deactivate a License" and supply the activation key AND the computer ID you got from the previous page, to remove old activations that would allow new ones.