With the MP4 update last year, we made it so that you can update the map file on the map page, and it retains all the replays, as long as the UID of the map doesn't change.
I just tested in-game by creating a Lagoon map with no custom music. Saved the map.
UID: FZZp82KAopLSIr5rcrZb78guyK2
Reopened the map in the editor, selected some custom music. Resaved the map to a new folder.
UID: FZZp82KAopLSIr5rcrZb78guyK2
So as long as the validation and map name remains the same, the UID should as well. And you should be able to update the map file without losing the replays on the track page.
Therefore I'm marking this one fixed.