Can we change the closing date without reverting to the buyer for a 're-sign' ?
Not really. The contract shows price, closing date and contents.
I suggest that you request an extension of time now - before signing contracts - if they refuse, decide whether or not you're willing to either lose good faith by subsequently pushing out the closing date or pull out and hope for another purchaser. If they say yes, thats great.
It is hard when you're in a chain because there is a large element of good faith and trust involved. Everyone is trying to please themselves and get their own stuff organised.
mf