I play in a prog-metal band now, my biggest fear was that I wouldn't be good enough. I joined and looked at the written basslines, learnt them & struggled with them then asked if they minded me editing them slightly. Now slightly simplified in most places (and more complex in others) and the songs have a LOT more punch!
Just finished editing the bassline to a slower song which was really melodic but sounded really busy, now its simpler and really sticks out in the mix. Guitarist couldn't be happier!
Good band this! Like others have said it's what the song demands not what you want to play.