High Sierra converted Keynote and now can't open in El Capitan

So, I go back and forth pretty seamlessly between two computers, a MacPro 8 Core from 2008 (El Capitan) and a MacBook Pro (High Sierra) from 2012 using Dropbox and/or iCloud. Most documents work on both computers without a problem. Office, iWork etc. Just tonight, however a Keynote I created on El Capitan and then opened in High Sierra now no longer works in El Capitan. The High Sierra version of Keynote never once notified me I would be upgrading the document with no chance of going back. Am I doing something wrong? And am I crazy to think that Apple would not break compatibility without at least letting us know? I think there should be at least some backwards compatibility. Thanks for any suggestions.