Yes, if the old site is running 6.3, then codes for 7.0 (such as the text alignment codes) won't work for it as they where added later.
Another good example is the Spoiler tag, which will be added in 7.2; it will not work with prior versions without being added into the coding by the owner of the board as a modification (should one even exist for that version).
Also, 6.3 is fairly old and insecure, your admin should consider upgrading to 6.5.5 at minimum.
