Consider switching to SMIL timecode for project files
MLT has an option to save timecodes in SMIL format ([[hh:]mm:]ss[.fraction]) instead of frames. This could make it much easier to convert a project to a different fps.
This is enabled by setting the "time_format" property on the xml consumer to "clock" (see consumer_xml.c line 746 and mlt_types.h in MLT).