The demo and the full-game are technically the same title or “Product”, which is the definition we use in our EULA.
General rule is that you must submit the Release Form before any distribution of the game that will generate revenue. As per our EULA, in the Royalty Addendum:
Before you Distribute a Royalty Product that will generate revenue, you must give Epic notice of your intended Distribution (“Notice of Distribution”) as early as reasonably possible and in any event prior to the Distribution of the Royalty Product. Likewise, if you plan on beginning to generate revenue from a Royalty Product that you have already Distributed, you must submit a Notice of Distribution as early as reasonably possible before the Royalty Product is used to generate revenue.
Hope that helps!