According to Nina Jacobson, the producer, in this answer on Quora:
Practically, Phil had shot about 80% of his scenes. What we had to do with the remaining 20% was to give two key scenes to Liz Banks and Woody Harrelson. In MJ1, Effie gives Katniss Cinna's design for the mockinglymockingjay uniform instead of Plutarch, as was originally scripted. In MJ2, Haymitch reads a letter in a scene that, in the book and the original script, takes places between Katniss and Plutarch. Without a doubt, Phil would be a bigger presence in the movie if he had stayed with us. I miss him at many levels.