I personally don't like it when a game comes to a screeching halt and rips me out of the experience by reminding me that I'm being a cheapskate
If possible, I would try to hide these areas and weave them into the storyworld as "mysterious", "forbidden", or "forsaken realms", etc. and then when users pay for the full game, you actually show these being unlocked or tie why they are open into the story somehow.
The other alternative is to have the paid version's information OUTSIDE of the main game, but positioned on the title screen or in a HUD or menu somewhere. It's OK for that type of 4th wall breaking information to be there, because the player is in a different mindset at that time.
Those are my thoughts on it anyway.