Once the service stops using Twitter, it could be considered a full "switch" I suppose.

I rewrote the title a few times, couldn't settle on anything. Updated it again now (criticism was valid, I originally posted with "Japan Earthquake Alert App switches from Twitter to Mastodon, already has 488k followers")

Probably should have just included the relevant part of the article

We are moving forward under the premise that our operations should move from platforms managed by other companies to those within our control. Our app of course works on Apple and Google but these are different in terms of API constraints. On the other hand, distribution on ActivityPub is where we can manage our content completely independently. For this reason, in response to requests to move our operations onto different platforms will eventually only result in the same outcome as this time and therefore will not be considered. As NERV posts are still accessible via social media linked with ActivityPub, we plan to continue sharing there.