I'm nowhere near an expert in the CC game, and this is usually the only type of question that a blogger could help with...

TPG had this up on his post from the last time the Explorer card bumped up to 50k:

”This new cardmember bonus offer is not available to either (i) current cardmembers of this consumer credit card, or (ii) previous cardmembers of this consumer credit card who received a new cardmember bonus for this consumer credit card within the last 24 months.”