The original OGL granted “perpetual, worldwide, non-exclusive license” to the Open Game Content (commonly called the System Resource Document) and directed that licensees “may use any authorized version of this License to copy, modify and distribute any Open Game Content originally distributed under any version of this License.” But the updated OGL says that “this agreement is…an update to the previously available OGL 1.0(a), which is no longer an authorized license agreement.”

I'm pretty sure that once you grant a perpetual license it cannot be revoked. Perpetual does mean "forever" after all. My suspicion is if they tried shut down competitors using material under the original license a court would say that the revised license only extends to WOTC content introduced after it went into effect.