Discussion point: When a band changes vocalists, is it really the same band? For me, vocals are the most important part of a band and I have trouble getting used to different ones. Maybe it's because I'm so picky about them.
The essence of the band, the thing that makes it 'it'? Or the legal trademark that allows a certain group to call themselves with a certain name? For the sake of discussion I assume we're going with the first.
Let's take Nightwish as an example. You have symphonic metal thing with fantasy lyrics and somewhat operatic female vocals. Kick Tarja out, get Miss A in. Now you have a symphonic metal thing with fantasy lyrics and somewhat more rock female vocals. While it's certainly different, I really don't feel that the band as a whole changed. The over arching style remained largely the same. Vocals are different, yes, but they're only one facet of the whole that comes from the music, lyrical themes et al..
Now let's look at Turisas and compare the
first and the
third (thus far their latest, I believe) album. There's no significant line-up changes that I know of, and at the very least the vocalist stayed the same. Debut has (to my ears) bit heavier folk metal with some clean vocals and some more aggressive. The third feels significantly less folk-y, more distant from its roots. I'm at a loss for words here, but I'll say it feels less heavy. And the vocals are fairly clean throughout the album. For me it's in many ways like I'm listening to two different bands when I listen to their first and third in succession. I don't mind it but it definitely doesn't sound the same. The changes in overall style are significant enough that I see a very clear and distinct difference between the first and the third albums. "The band has changed", "It's not the same band" or "It's a different band" are phrases that one could utter. Despite the vocalist staying the same, many things that made Turisas 'Turisas' in their first album have changed by third their album.
In short: I think the overall style is more important than a single vocalist when considering whether the band "has changed" or not. For some bands the vocalist is without a doubt the single most important member, but I try to consider the bands as a whole when thinking about these. Case in point: for me Queen is as much about May's guitar work as it is about Mercury's singing, had either one of them left the band I would have considered it an irreplaceable loss.
Sabaton with a convincing female singer, assuming the musical style and lyrics stay the same? Yeah, It's Sabaton.