You are making this way more complicated than it needs to be. Columbo is practicing at LT "this year" because we don't have anybody than can step in at LT if Tucker goes down. Bill has said that Pettiti "will not move to LT if an injury occurs there". If Pettiti goes down, Tucker can move over to RT in a pinch, but if Tucker goes down, then Columbo is who will step in which is why he is practicing there.
Now what we do this season does not mean that is what we will do during the offseason or training camp. Columbo may very well be moved over to RT, it just won't happen this season because of our O-line situation.