This is an issue which Craig Morgan Robson think about a lot, as we consist of a Scot, a Northumbrian and a Southerner (albeit with some Yorkshire roots, but they are buried a bit deep!) who sing together in harmony.   We are very keen on singing songs from each of our "home" regions, which potentially poses a problem of accent. I would never attempt to sing a song from the North East as a solo, but when Carolyn leads a song in which she uses her native accent, I know that I adapt to match her vowel sounds as closely as I can, as to me that is part of creating the blend of sound that we want to make. We have tried singing songs (as an example at a workshop) where we each maintain our "original" accents and we now find it almost impossible, as we so used to tuning in to each other.

However, this doesn't feel so much like "changing my accent" as "matching the sound of my fellow singer".   I don't think I could cope with "putting on" a different accent for singing regional songs as a soloist, and I have to confess I find it a bit grating when I hear it from others. I don't count those who are have two accents - the one they were brought up with and the one they had to learn for school or work - I think they are just lucky to be "bi-accentual" if there is such a term!
Saro