Know you are calling for two specific persons to respond, but thought I'd share ... going through this migration 1.9.19+ to the goal of 2.4.4+ (maybe 2.5.x) right now.
Site migration to 2.2.10 did bring across users and courses ... but along with everything ... Legacy files (all courses).
Recently, there have been other discussions about restoring 1.9.19+ course backups to 2.4 (or 2.5). At one time, if one restored 1.9.19+ courses in a certain fashion to a 2.2.x and a 2.3.x, one could rid the course of Legacy. However, now, due to updates to 2.3.x and 2.4.x no longer possible.
Think I've asked asked about Legacy files before ... like how long before 'legacy files' support is dropped. Realize cannot support 'legacy' forever, just wanted to know if backups of courses that did contain legacy would be totally useless at that point ... imagine they would be.
Do I have an answer for ya? Well, considering the current state of things ... migrate to a 2.2.10 and do the 'special' restore to get courses out of legacy at that stage of the 'march'. Once all the courses are out of legacy, easy to turn off legacy site wide and git update to 2.3 -> 2.4 -> and 2.5 then.
If you have a few courses to do, not really a problem. If you have 100's, it will take a looooooong time for one individual. To the best of my knowledge there is no command line script or work-around or add-on/plugin to assist.
'spirit of sharing', Ken