Movavi Video Editor changed formats for projects several times and both .mepx and .mepj files are exactly this, projects from different version. In case of mepx to mepj conversion, this is merely a resaving of older project to the newer one. When you open old .mepx files in modern Movavi Video Editor and save the project again, it will be saved in the latest variant. No dedicated converter exists for this, nor it is needed.
Updated: October 11, 2023