Hi, your post was a little unclear to me. I know, how to change the file system of a partition, BUT IT WILL WIPE EVERYTHING ON THE PARTITION! Also, what exactly is in sda3? Is that where your current distro is installed, is that where you want the new one installed, or both?
If you're trying to DEcrypt sda2, then I believe cryptsetup should have a tool to do so, but I don't know because I've never used it. Also, don't bother doing apt-get remove cryptsetup. It won't decrypt anything, it will just remove the software. On another note, why do you want to use truecrypt instead of cryptsetup?