Do you think this is necessary? You can import the existing tracklist on the "new release" and then remove it from the old one; in theory retaining all the recording links and such. And since a medium is not (currently) an AR-able entity, there's not much editing duplication - I guess you have to set the disc title and the medium type again though.

I think this is very important, for it would make it possible also to "split" releases that were incorrectly merged in the NGS conversion (like the "Rock Furore" series)

I think this is a reasonable request to keep in mind when designing any rewrite of the release editor, at the very least; I'm personally unsure of its general utility (and would thus probably put it lower-priority than a lot of other features), but it's certainly worth keeping track of.