Thought experiment:
Suppose I auditioned as a lead singer for a local classic rock cover band that had been around for several years and had a decent fan base and did about 5 or 6 gigs a month, but they needed to replace their singer.
I nail the audition and the band says they think my voice is fantastic and I'm exactly the front man they're looking for and they invite me to join.
I tell them that I'll agree to join, but I have one condition. After looking over the 50 songs on the setlist they gave me, I want to change the key of 10 of those songs. I explain that those songs are tough for me to sing and I will sound better and there will be less chance of my voice pooping out if the guitarist, keyboard player, and bassist all transpose key on those 10 songs.
I'm wondering what their reaction would be.
1) "Cool. You're the singer and our fans care more about what you sound like than the rest of the band. We can do whatever it takes to accommodate you. The guitarist will buy a 7 string guitar and the bassist will buy a 5 string bass. We're professionals."
2) "Get lost."
3) "Maybe you should eat more carrots."
IMHO there is a natural tendency for people to resist change. I think their willingness to put in an effort will vary depending upon circumstances:
How long has the band been looking for a vocalist? If it's been a long time, then they are more likely to put in an effort.
How awesome are you? If you are truly remarkable they will be more likely to put in the effort. Putting in an effort may involve changing keys, or choosing/learning new repertoire.
How competent are the musicians? Highly competent musicians will be more receptive to change, and most likely they are used to dealing with it. There may be some negotiation/compromise though. For example instead of dropping the song a whole step, maybe it needs to drop a minor third so it goes into a better key for the guitarist. This can be driven by the nature of the instrument. (In my experience, it's pretty rare to find an electric player who uses a capo)
In my career, it was fairly common for us to have personnel changes; that is just the way it works in the military.
We generally had little to no input on which personnel were assigned to an ensemble, so a new vocalist usually meant major changes to the repertoire. But the reality is we tended to rotate repertoire to some degree even when we had the same personnel for long periods of time. The band I was assigned to lead played a mix of classic and top 40 country, so we continuously added contemporary hits when they were fresh, and then dropped material that had become stale. We also occasionally rotated some of our classic country repertoire.
An observation: If you have a hotshot guitarist you will choose material that showcases the guitarist's chops. When the guitarist moves on, you may have to drop material because the new player may not be able to pull it off. Or if it works, you might modify the arrangement so it no longer features the guitar.
When you get a new vocalist, you generally totally rework the repertoire to suit the vocalists skill, voice, and personality.
If you wanted to maintain the exact same repertoire, then you need to find another vocalists who is very similar to the person they are replacing. This is typically not an easy task in my experience. It's hard enough finding a good vocalists, much less a good vocalist with a very specific skillset.
For the record I remember making adjustments to accommodate the vocalist long before I turned pro. Also the reason I wanted to learn 5-string bass had little to do with accommodating the vocalists. I wanted to be able to play songs that were originally played on synth or 5-string.
My first bass was a Ric 4001, and I remember occasionally detuning to drop D to cover some songs. It was a hassle because then I sometimes had to play other songs in drop D, until I could get to transition where there was time to retune the instrument without interrupting the flow of the show.
I believe the original was in D. 
