I didn't mean to suggest this move was about making him a RP now, rather, given the logjam of starting pitching ahead of him, he is a better candidate for the pen than some of the other pitchers. Another issue for Anderson is, all else held equal, they'll promote Irvin, Romero and other LHP ahead of him b/c they don't have a solid LH SP candidate right now.
They like pitchers to stay starters as long as practical, because they get more batters to work on their command and secondary stuff.
With Anderson, it'll probably come down to stamina (he's not a big guy), whether his arm gets stronger further from TJ surgery, and the progress of his secondary pitches.