Users for whom Lightning Experience is enabled will be able to use the Switcher feature, located in the upper-right side of all pages, to toggle between the user experiences to access pages as necessary. The attributes (like field order and field-level security) will be the same in the two UIs.
If a Lightning Experience user sends a link (Chatter post or email) to a Salesforce Classic-only user, the page will not be found. Salesforce Classic links are interpreted for routing to the Lightning Experience UI, but not the other way around.