Nintendo has announced that a new software update will become available for the fantastic Super Smash Bros Ultimate on Monday, 18th October. The software update will accompany the final Super Smash Bros Ultimate downloadable content character which is Sora from the beloved Kingdom Hearts franchise from Square Enix. While the patch notes have yet to released Nintendo advises converting your reply data in the brawler as replay data from previous update versions could become uncompatible. Super Smash Bros Ultimate is out now and we will publish the patch notes for Super Smash Bros Ultimate Version 13.0.0 once they become available,
Super Smash Bros Ultimate Version 13.0.0 coming on 18th October

Wait, how is this news? Sakurai confirmed it in the Smash Presentation.
It’s more so a reminder to convert your replays before it’s too late
As Lucky Bunny said, it’s pretty much a reminder :)
Yeah its good to remind someone. :D
Why is it not possible for them to make the replays still work?
Because the replays by default aren’t exported videos, they are literally replaying the actual saved data of the match.
Basically, if there are moves that get buffed/nerfed in this patch, the replays where they were used won’t play out the same way as they originally would have, meaning the replay is going to be inaccurate. Even smaller buffs/nerfs could make a big difference over the course of a whole match.
But if you convert it to video, you won’t have to worry about any of that, which is why they suggest exporting them.