This program converts APNG animations into animated GIF format. Wu64 quantization is used for true-color files. Transparency is handled either using a threshold, or composed over chosen background color. CLI version is OS-independent, GUI version is available for Windows.
Another converter: http://sourceforge.net/projects/gif2apng
This program converts GIF animations into animated PNG format. Usually it makes the files smaller. CLI version is OS-independent, GUI version is available for Windows.
Another converter: http://sourceforge.net/projects/apng2gif