From my experience (and several others who have posted here and/or on the Vegas Message Board), your Diamond status will expire within a few days of your Founders Club expiration date (or within a couple days after FC calls to try to persuade you to renew).

My experience:
After I let my Founders Club membership lapse, I was called twice (by FC) within a week and offered some incentives to renew. When I declined the second offer my Diamond status disappeared 48 hours later. Others have indicated the same (even if they weren't called by FC).

But, good luck, maybe Founders Club/Caesars have changed their policies(?).