The easy applications to convert to open source are those that the developer of the application is the user of the application. The easiest example is Bitkeeper to GIT conversion. The person who wrote GIT is the same person who used Bitkeeper. Therefore he knew what would be usable for a developer.
GIMP vs Blender is the counter example. Both GIMP and Blender are targeted at artists and not developers. The author assumes the awesomeness of Blender is inevitable. But GIMP is pretty bad and shows no evidence that it is getting better. Yet - when major animation studios decided to adopt Blender they dedicated UX resources to Blender to make it more usable for artists.
When the developer is not the user, you need that extra step.