Earlier this week, Matt Simonini wrote about how moving from the NL to the AL hurts pitchers' performance. In the post, Matt showed how Josh Beckett's statistics from his final season with the Florida Marlins were better than his first season with the Boston Red Sox.
From my perspective, it doesn't matter if a pitcher jumps from the AL to NL or the NL to AL. Switching leagues benefits the pitcher immediately. Batters in the opposing league have likely never faced this pitcher before and have absolutely no idea what they are about to see. All pitchers are scouted by teams, but watching film and mental reps cannot prepare you as well as stepping in there and taking some pitches.
AL to NL
The exemplary pitcher used for jumping from the AL to the NL is none other than the recent 300 game winner himself, Randy Johnson. In 1998 with the Seattle Mariners (AL), Randy Johnson went 9-10 with a 4.33 ERA, 1.28 WHIP and a 3.5:1 K/BB ratio. In the same year, Randy Johnson finished his second half of the season going 10-1, 1.28 ERA, 0.98 WHIP, 4.5:1 K/BB with the Houston Astros (NL). In 1999 Randy pitched his first full season in the National League with the Arizona Diamondbacks (just to show it wasn't a fluke), going 17-9, 2.48 ERA, 1.02 WHIP, 5.2:1 K/BB.
NL to AL
In only his second major league season in 1989, Randy Johnson went 0-4, 6.67 ERA, 1.85 WHIP, 1:1 K/BB with the Montreal Expos (NL). In 1989 Randy Johnson finished his second half of the season 7-9, 4.40 ERA, 1.44 WHIP, 1.5:1 K/BB with the Seattle Mariners (AL). Not to say that his numbers were great with his initial start with the Mariners, but they were much better than what he had done in the NL in 1989. In his second season in the AL with Seattle, Johnson went 14-11, 3.65 ERA, 1.34 WHIP, 1.6:1 K/BB and also made his first All-Star appearance.
Benefits From Switching Leagues
Moving from one league to the next gives a pitcher a new opportunity to face batters that have never seen him before. Switching leagues also gives a pitcher that sense of urgency to prove the skeptics wrong and show them they've still got what it takes to be an effective major league pitcher.
Though it's a small sample size, Randy Johnson shows that switching leagues is effective and absolutely beneficial for the pitcher. Using the same pitcher switching from each league to the other keeps the pitcher constant in the experiment. Although Randy Johnson was much older when he switched from the AL to NL, it still shows that he pitched better in the year and a half in the newer league than he had done previously with the original team he had opened the season for.
Statistics from Baseball-Reference.com