I was thinking about the Raspberries. On their first album (Raspberries), Eric Carmen played bass while David Smalley played rhythm guitar. By the second album, Fresh, Eric Carmen had switched to play rhythm guitar while David Smalley was now on bass. I can think of a couple other bands where something like this happened. I have seen a post similar to this, but I am not looking at a member who switched because someone left (say Paul McCartney taking up bass when Stuart Sutcliffe left The Beatles), but members who swapped duties. Anyone else know of any others?