Time to time, there is a high need in preserving either the existing vocals or the existing music during the regeneration.

Here is a very good example where I have spent a lot of attempts and time to generate the desired vocals and where I'd like to regenerate the music:

The entire regeneration (i.e. vocals+music) is not an option for me since a lot of time has been spent to generate at least the desired vocals. Now I want to regenerate (change) only the music, keeping the vocals as is.

Sometimes there is the opposite situation, when the music is completely OK, but the vocals are not that good. In such case, there is a need to regenerate only the vocals, keeping the music as is.

This is an absolutely "must-have" feature for me.