Describe Jonas' escape plan? Why was Jonas forced to change it?

In Chapter Twenty, we learn that Jonas plans to leave in two weeks, the Giver will stay behind to advise and share his wisdom with the community. He plans on leaving the night before the ceremony, hoping his absence won't be noted, that the ceremony will provide a distraction and allow him to escape. The Giver plans to order a car and driver, sneak Jonas into the car, and later, tell everyone that Jonas was lost in the river.

Jonas' plans are unexpectedly changed when he learns that Gabriel is going to be released. In order to escape and save Gabriel, Jonas takes Gabriel from the Nurturing Center and escapes. Jonas and Gabriel ride as far as they can on his father's bike. They sleep inbetween the darkness of trees, and Jonas provides Gabriel with good memories to keep him calm.