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
Starts elsewhere: 1,207
(including 1,206 at 1B or 3B)
Year/age when last caught 100 games: 1967 / 26
B.J. Surhoff
Career: 1987-2005
Starts at catcher: 648
Starts elsewhere: 1,463 (including 935 in OF, 280 at 3B)
Year/age when last caught 100 games: 1992 / 27
Brian Downing
Career: 1973-92
Starts at catcher: 610
Starts elsewhere: 1,544 (including 790 at DH, 750 in OF)
Year/age when last caught 100 games: 1979 / 28
Craig Biggio
Career: 1988-2007
Starts at catcher: 392
Starts elsewhere: 2,331 (including 1,959 at 2B, 352 in OF)
Year/age when last caught 100 games: 1991 / 25
how mauer compares
Career: 2004-????
Starts at catcher: 885
Starts elsewhere: 243 (including 188 at DH, 54 at 1B)
Year/age when last caught 100 games: 2010 / 27
Source: baseball-reference.com