I use my ancient ReprojectImage toy ( http://www.ridgenet.net/~jslayton/ReprojectImage.zip - Windows only) to push things from various projections to Equirectangular.

Hugin has been mentioned as a way to move thigns from one projeciton to another, but I haven't used it.

I would think that you should be able to use GDAL or QGIS or any number of GIS tool sets to do this. In my opinion, though, those tools are a little peculiar in mindset - not surprising considering their purpose, though.

Try searching here at the guild for "ReprojectImage" and you should discover threads similar to this one.