Does your band ever switch instruments or play with more or fewer people than usual at select shows?
My band played as a three-piece tonight with me on drums/vocals (I usually sing and play guitar/keys), our guitarist/vocalist, and our bassist (who kept their usual roles). We had a couple friends sub in on guitar and vocals for a couple songs too. We typically play rock of all sorts, but for this show we stuck to fast, high-energy stuff that could be played with one guitar (and we threw in an impromptu Bulls On Parade cover at the end).
We played as a three piece before at a school event, since only three of us go to the same school (drummer on drums, guitarist/vocalist on guitar/vocals, me on bass); we've also played as a four-piece with me subbing in for our drummer who was sick.
Come to think of it, at that school thing, we also switched it up and had me on drums/vocals and our usual drummer played saxophone.

Any similar stories?
Never for serious. Like we'll occasionally jam on other instruments at practice.

Though one of my bands does strip down a bit. It's a band with a Sax player, but she's a professional musician so she's often busy, when this is the case we play as a four piece.
In one of my bands I alternate between bass and lead guitar. There's a rhythm guitarist so I just play whatever sounds best for the song. So some songs have two acoustic guitars and drums, others have bass drums and guitar depending on what I'm playing.
And no, Guitar Hero will not help. Even on expert. Really.
I've done gigs with a band called the 3 Amigos (I'm on bass and vocals) where we all swap instruments one way (I play drums, the guitarist plays bass and the drummer plays guitar) then swap the other way (I play guitar, the guitarist plays drums and the drummer plays bass) so that we've all played each others instrument after just two songs.

It's lots of fun, and if you can do it well, the audience just loves it.
most i've done is switch from electric to acoustic and my bass player grabbed a harmonica. but then again we do have plans to switch guitar and bass roles for a song hes gonna sing.
no sir away a papaya war is on
I do guitar, lead vocals on a song, keyboards on a song and bass. The keyboardist plays guitar a 1/2 our songs. Female lead singer plays keyboards on a few songs. It's good to cover several different bands/song types. They key is to group songs together the best you can so that people are not changing instruments (long breaks) in between every song.
One of my bands has 3 core member: Me (drums and vox) and a guitarist and a bassist. Then we have a rotating supply of musicians who play a variety of instruments, including trumpet, accordion, additional guitars, freestyle rapping, and drumming for me so I can focus on singing, to be used as the songs require. But almost every one of our songs can be played as a three-piece.