Both .mepj and .mep files are projects from Movavi Video Editor, albeit from different versions. It's usually possible to re-save older project to newer ones, but the opposite is rarely possible and this is the case of mepj to mep conversion.
If you still think this should be possible and need it for some particular purpose, it might be best to reach out to Movavi support for help.
Updated: October 12, 2023