In short: Changing the tags in MP3Tag doesn't change anything for a song that's already live with the distributor.
The tags you edit in MP3Tag are stored in the audio file. A song that has already been released on streaming services or stores keeps the data it was submitted with.
If you want to change the data of a live song:
- The release has to be removed from the distributor and resubmitted.
- Be sure to note down the ISRC beforehand. It has to stay the same, because it's the identifier of your song across all stores and for royalties.
- The best thing to do is to contact support.
About the ISRC in general: You shouldn't change an ISRC that has already been assigned. It's the permanent identifier of your song.