Hi, what should be done in case an artist modifies a song after releasing it?
In this specific case: Release “nadie sabe lo que va a pasar mañana” by Bad Bunny - MusicBrainz
The artist changed the song EUROPA 
to EUROPA 
And also changed the lyrics and duration (just 1 second), but kept the same ISRC. Should I create a new release and recording and add the proper disambiguation, or modify the existing ones?
Thank you.
That sounds like enough difference to need to have a new release. It is now literally a new recording as it sounds different with new lyrics. Even the change of title would have been enough of a reason.
ISRC is just how they get paid. Often not unique.
At it’s heart, MB is about the music and packaging that goes with it.
well for example the dance-pop group “The Go! Team” released their first album Thunder, Lightning, Strike (2004) with uncleared samples, and the label were forced to re-issue the album (under the same title) with new cleared samples; so in MBz you’ll find two recordings for most tracks on that album for the pre and post adjustment songs.
Just use the disambiguation field to explain your case, and if necessary annotation for further explanation!
Understood, I will create a new release. Thank you @sound.and.vision @IvanDobsky 
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Oh, and if the artist modified just the cover, should I also create a new release or keep both covers in the same one?
new cover art also gets a new release. and set the release date as the date of the change if known, or leave it blank if you don’t know