Was it refurbished by Apple or a third party? How long ago did you purchase it?
I’m a former Apple Store manager and it would really be at the discretion of the Lead Genius. The liability or risk would be that in the process of replacing the battery the back is damaged more and/or affects the operation of camera or other functions.
Every person’s initial reaction is that they waive any liability etc. but Apple has no formal policy for doing that and again would be at the discretion of the Lead Genius.
I suspect if you sent it in for a replacement by Apple, they might refuse the repair without replacing the back. It’s going to be hit or miss.
Sorry I don’t have a more definitive answer for you. Maybe a community member will share their experience with repairs/ replacement when the back is cracked.