Many of my songs translate well to other instruments, as I consider the DNA of my music to be in the melody. Thus in my songs, timbre is one of the least important aspects.
In my operas, timbre is specifically used for psychological insight into the characters. Thus, I am less likely to allow any changes (although sometimes variety is already built into the vocal lines, i.e., the role of Hester in The Scarlet Letter, has many options for the soprano).
Sometimes it can work to change the accompanying instrument — sometimes not. It all depends on the song and how things sound with the new instrumentation. I am always willing to consider any requests.