Why is "The Giver" ready for a plan for change in Chapters 20-21?
The Giver has believed for years that "things must change" in the community, but there had seemed to be no way for that to be accomplished. Through his sharing of memories with Jonas over the past year, he has become even more convinced that things should be different, and, inadvertently, Jonas himself has finally given him an idea of how this might occur.
When the Giver had told Jonas that Rosemary, his planned predecessor, had been released early in her training, Jonas had wondered what would happen if something happened to him while he was in training - if he were to have an accident, perhaps, like drowning in the river. The Giver had explained that his memories would return to the people, and that it would be very difficult for them to deal with them. Upon further thought, however, the Giver entertains the idea that if he himself were to be there to help them through the transition, things might work out (Chapter 19).
Now, "for the first time, (the Giver) think(s) there might be a way". The Giver has already given Jonas a wealth of memories, and if these memories could be given back to the people, with proper support they could potentially become a complete, feeling community once again. The plan the Giver devises is dangerous, but it is possible Jonas believes in what he is doing enough to undertake the risk, and so he and the Giver set the plan in motion, to be executed the night before the coming Ceremonies (Chapter 20).