I agree with Q, leave it alone if the composer is living.
In the early decades after he/she/its death, think real hard about making changes, and if you're 'correcting' something you better be sure and corroborate your brilliant changes.

As for ancient, dusty, endangered or unsung songs that need help to be singable and performable, change away if it will give them a new lease on life, THAT's why there's a folk process. An unsung song does no one any good.