I just noticed in an emailed ad that they have "reopened the upgrade pathways" to allow X8 to be an upgrade to as far back as v1!
That's something that I have never seen, and could really be incentive to those with an old copy of v3 from Costco kicking around to re-visit.
Now, whether that old version needs to be installed on the same machine or one merely needs the key or serial number, I don't know.