At least in the UK, from what I understand, if you make a "not actually new" release (by adding something like a director's commentary), it has to be re-rated by the BBFC. There's also the case of, IIRC, "The Specialist" which was a 15 in cinemas but an 18 when later released on video.[1]
(None of which is relevant to the YouTube argument, mind.)
[1] https://www.bbfc.co.uk/release/the-specialist-q29sbgvjdglvbj...