Where is my old form data?

Upon downgrading my membership, it clearly stated, 'Your account will be downgraded automatically at the end of your current billing cycle: March 27th, 2018.'

Why do I not, then, have access to my 1000+ submissions from within that billing cycle - for which I have already paid?

I believe the submissions in excess of 500 were hidden before your previously created subscription.  They should have been restored immediately when you subscription but it appears they were not.  I restored the hidden submissions and all data should be available until the end of your current paid period.