Well, seeing as you're the only serious reply, I guess we should have a conversation.
For normal (proprietary) licenses it's already been established that a copyright owner can revoke the license at any time simply by giving notice to the licensee. (Wood v Leadbitter).
They can legally do a bait and switch, handing out permissive licenses, waiting a few years, then revoking all the licenses and demanding payment from anyone who continues to copy or modify the software. This is established law in Australia (Computermate Products v Ozi-Soft), and the only law prohibiting it in the US is UCITA, which thankfully never much made it (as it has much worse effects, namely clickwrap licensing).
Ultimately it would take someone with deep pockets to do this.. and they'd have to be outright malicious towards Free Software (thus, Larry). They'd preferably want a product that was dual licensed in the first place, never really had much of a development community, and had a strict copyright assignment policy for contributors (the two tend to go hand in hand).. that way they can simply declare "the only license available for this software is now [the proprietary license]", and because there was no doubt that they are the sole copyright owner it would be sufficiently ambiguous whether or not they were referring to all versions of the software or just the future versions.
Their sales team would be instructed to inform customers that no, there is no GPL version of the software anymore, and yes, you do have to buy a license to install the software. At that point they're a proprietary software company and they will start treating their customers like proprietary software customers - that is, they'll start showing up at the doorstep to do software audits. That's when they start putting the hard word on customers to "upgrade" to the proprietary licensed software.. if the customers resist *on the grounds that they don't need to pay for old versions*, then the legal fangs come out and we find out if this whole GPL thing has been built on a foundation of sand.