Classic Started again after 12 years with the wife.

So my old account still existed and I renewed the monthly subscription. The wife made an account so we are currently a duo in the 30's. I have a few other characters in the 40's that I don't remember how to play and she only has the one character. We recently figured out we are playing retail but not the full version. Hell I thought we were playing classic.. Anyway, if we buy the expansion do we have to start over or is it a continuation on the same server? What changes does it make pre 50? Appreciate any help. via /r/wow https://ift.tt/W4LQ2zi