Band Evolution

I was reading up on the Dire Strait's history on wikipedia and listening to "Romeo and Juliet" and I noticed their member list was quite diverse. Subsequently the question came to my mind, "At what point is a band not the same because of shifting members?" In the Dire Straits example it's not too drastic because they did keep the lead singer(Mark Knopfler) and bassist(John Illsey) but at the same time the other two to four members over the years altered regularly. They might play the same songs and try to imitate the style but the real feeling of a musical group is with its soul members. When the members are switching around ever year or so can you really say you enjoy the Dire Straits or did you just enjoy the Dire Straits during their 1982-1984 era? Personally, I don't think a band should rightfully retain the name if it's not the true heart and soul of the group or the founding members. Realistically they can't just decided to change their name when they've gotten considerable success but I don't think they can really be considered that band without at least mostly original members so they retain that real emotion that the creator put into it. Maybe nothing can really change, it's a fact of life musicians move around and find other opportunities or have needs they need to fill and it's understandable, it just doesn't seem right to refer to a band as the same name when the current members barely resemble their predecessors ten years prior.