Thanks. I actually just figured it out. The answer is Yes, but it isn't clear, even after confirming with Apple tech support.
After upgrading from iOS 9 to iOS 10, opting out of any upgrades to iCloud Drive, and then launching the Notes app, a screen came up with only one option: "Upgrade Now". I had my latest iOS 9 state backed up and had Airplane Mode On as a safety to prevent any possible issues with iCloud & syncing. After tapping this option, another pop-up appeared which then gave me an option for "Not Now" which I had to select to maintain sync compatibility with Notes and OS's prior to El Capitan. This was the button i was looking for but it did not appear until this stage in the process.