You must use the "Deactivate" button in the Addon Manager menu INSIDE THE SIM, on the old PC, when everything is still activated, before disposing it/giving it away/reformatting.
Not that you HAVE to do this. The only reason to Deactivate, is to not consume an activation when reinstalling on the new hardware, but you have 6 of them so, if you cannot Deactivate for some reasons, for example because the old hardware broke down, or you reformatted before deactivating, it's not really a problem: you still have several activations left.