Well, There IS however a (seemingly) official way to play Terraria 1.0.6.1 (The update before hardmode). But I do recommend backing up your current saves before trying that.
Go to the location where the Terraria files are saved. (C: > Program files (x86) > Steam > SteamApps > Common > Terraria) There, you'll find a map called "Older versions". and you can find the maps of the very first version and version 1.0.6.1 of Terraria.
I found those files once while I was bored, and I've also decided to play them for nostalgia purposes, and they do work too!
I've no idea how and why those files are there, but these get downloaded when you download the official Terraria file, so there's that.