Well there's an album of new material completely recorded already. All they'd need to do is go back into the multitracks, mute/remove Dan's vocal tracks, and re-track with John. Whether or not they rewrite the lyrics or not would depend on if John wants to write and whether or not Dan had written anything that they can't/won't use anymore. I'd see no point in not using the MUSIC end of the tracks they've already recorded. They'd have to of course remix and have the album mastered again - which could cost quite a bit - unless they can keep all of the automation and mixing levels the same on the music while retracking vocals - then it'd be just a matter of recording vocals with John, getting the effects on them, and mixing them in at the right level with the music.