it's been done before
Joe Mauer's switch to first base is not a groundbreaking move. Here are five other good-hitting catchers who spent considerable time at other positions:
Yogi Berra
Career: 1946-65
Starts at catcher: 1,641
Starts elsewhere: 248 (including 246 in OF)
Year/age when last caught 100 games: 1959 / 34
Joe Torre
Career: 1960-77
Starts at catcher: 836