Here's a slighty different question.
Suppose BobCo uses KenCo's OGC (the Dreaded Hephaflumph) in their OGL/3.x CastleDeathskull adventure. BobCo intends to upgrade to 4e and the GSL KenCo doesn't. BobCo wants to upgrade CastleDeathskull, and gets KenCo's permission to 4e-ize the Hephaflumph. In the meantime, KenCo releases a new OGL product containing the Hephaflumph, while continuing to sell the original Hephaflumph products as well.
When BobCo releases Castle Deathskull 4, does KenCo now need to stop selling their stock of Hephaflumphs? If KenCo later revisits their decision and moves to 4e, do they now need to license the 4e Hephaflumph from BobCo?