I'm in a band, we've been together for around 7 months now. I was called in to be lead guitarist, we have a singer/bassist whom had written around 6 songs and wanted to fill them out with a full band. All good.

So effectively we've been playing these six songs for quite a while, had spent time modifying my parts, talking with the band and playing around until they were all good, and started performing them live. We were all happy with them, or so I thought.....

Now, 7 months later, I thought these songs were now put to bed, and formed part of our set, keen to expand it with new songs. I wrote one to encourage others along. However, now the bassist/singer has come forward and wants to change EVERY song. He claims that he's never been happy with them, that they don't reflect what he had in mind when he wrote them, and that he's growing as a songwriter. In practice this translates to slowing EVERY song down significantly, replacing electric guitars with acoustic ones.

This also means that they are now different songs, requiring the drums and myself inparticular to rewrite our parts completely. Also I don't enjoy playing them the new way, I was comfortable with the old ones and just expected them to form part of our set.

Considering the band momentum was already slow, now I will have to halt looking for gigs, because our set doesn't reflect our songwriter's "vision". This all came to a head when I pleaded with him not to do it, just adjust one or two and write songs more in line with his "vision". He just repeats the line "I'm prepared to not play the songs at all then play them crap!". I'm tempted to quit, I don't think his "vision" would be served by electric lead guitar anyway, which is why I joined the band.

And no, Guitar Hero will not help. Even on expert. Really.
I think you should just quit. Or at least wait and write some faster songs that fit your vision more than theirs. Or keep trying to talk them out of it. It's ok for one or two songs to be acoustic and slow, but to have a whole set of acoustic slow songs (when you joined for electric guitar) is wrong. If they refuse to keep the songs as is, and you don't enjoy them almost at all, then I think it best to quit.
Try to compromise. Tell him that maybe you could have an acoustic part of your set, and he could write new songs for that. That way, you could still do the same songs that you already know, but he could still do his new thing too. Singers have issues...
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you could try and reach a happy medium and maybe change one or two of them and leave the rest the same to?
Maybe he could write a new song....? While songs go through revisions you don't see Opeth redo their older work because the band is growing.... that songs are to reflect you at that point or to send a message accross.. its great hes growing as an artist but that song was more of an expression of him... then... hes gotta move on..
i'm seeing a quit.

If you can't solve your problems with each-other (and if the drummer feels the same way about this as yourself) i say take the drummer, and start a new band.
You don't go rewrite works just because you are growing as a musician/songwriter. Especially after the whole band has learned the song. Tell him straightin out or you will leave and take others with you.
If he wrote those songs, and you guys aren't able to write your own, you are at his mercy. If he's that much of a stickler, quit and learn to write on your own. What he's doing isn't wrong, it's his prerogative as the writer of the songs to keep adjusting them until he's happy, but if it's not something you can deal with, quit. You guys need to be on the same page, and compromising seems like it'll make everyone unhappy.
Haha, I can write songs. I even mentioned in my opening post that I wrote a song for the band. Additionally I helped arrange the existing songs.

After considering this problem for a while, I decided that there is a bigger, unhidden problem that he would want to slow all the songs down. After prying it out of him it was because he felt uncomfortable singing so fast, he felt he could sing them better slower. BINGO!

So I comprimised with him to re-write two of the songs to acoustic. For another song I'm going to help him rewrite the melody/lyrics to make it easier for him to sing. For another song I proposed we just build up the song as it goes through, so it's half fast half slow.

It is an interesting concept though, when does the song you wrote become the band's song?

I know one thing, if you are unhappy with how it's being played, let them know at the very start how you want the song to sound. And this is best done through recording the song and giving them a copy. At the start. Not after 7 months and countless performances of them. At the start.
And no, Guitar Hero will not help. Even on expert. Really.