how did you discover Dream Theater?

[Kwyjibo]

It must be really weird to go at a gig an find out there's a stranger in place of a band member

Ah, the wonders of the pre-internet era 

And when the band members were introduced I understood something like "Dirk Sheridan", it was only a few days later that I learned Derek's real name.

[Outcrier]

I was 14 or 15 (maybe) and i used to play hacked versions of Guitar Hero 2...
In one of them, there was this song called The Dance Of Eternity that i liked very much and made me download all the discography of this band called Dream Theater.
Then, i heard Octavarium (the song), Metropolis pt.1, full SDOIT, full SFAM and they instantly turned in my favorite band at the time.
Nowadays, at the age of 20, i don't listen to them so much anymore (and progressive metal in general) but i always gonna like them 

[Lowdz]

It must be really weird to go at a gig an find out there's a stranger in place of a band member

Ah, the wonders of the pre-internet era 

I remember going to see Alice Cooper on the Hey Stoopid tour expecting to see Vinnie Moore on guitar only to find an imposter in his place- Ryan Roxie I think. Not a good trade and it spoilt the whole gig for me as VM was (and is) a top 5 guitarist for me.
No surprises in the internet age.