Alter composite index?

Is there any way to alter composite index?

No it is not possible. This would require a complete re-index of all docs in that table in order to also index the existing values of the column one removed/added to the index.Adding a new column would be different as this column has no existing values. So for this special purpose we could just change the index definition. You could create a feature request on github describing your use case. Also any contributions are very welcome.