I too would highly recommend spending the time in town, but unused fully flexible tickets are certainly refundable - people change the plans after checking in all the time. However, you should inform the ticket counter at the lounge that you will no longer be traveling on that flight and have booked a flight on another airline so they can offload you. Otherwise, there will definitely be complications with refunding the ticket.

A ticket isn't considered to be "used" until the segment is actually flown - the same reason you don't get miles for checking in for a flight but not boarding.

Also, to avoid complications in case they review your travel documents for compliance with visa or return ticket requirements, make sure you book a ticket to a country you can enter - in many cases you will need to book a round trip.