Rarely is this switch reversible. But it has been done recently, offering encouragement to guys like the Cubs' Ryan Dempster and the Phillies' Brett Myers, who are trying to rejoin the every-fifth-day world.
The U-turn king is John Smoltz who, due to team need and his own elbow issues, took a spectacular 3 1/2-year detour into the Braves' bullpen before resuming his life as a starter in 2005.
Adam Wainwright made room for himself on St. Louis' 2006 World Series championship staff as a reliever -- advancing to closing for the last month, including the postseason, after incumbent Jason Isringhausen gave in to a hip injury -- and returned to his "day" job in 2007 as a 200-inning starter.
Kelvim Escobar has had almost as many pitching lives as a cat: closed, started, closed for the Blue Jays, and has been a mainstay of the Angels' rotation for two years.
Given the science into which pitching has evolved, these are not simple switches. Different demands are placed on the arm, and on the pitchers' repertoire.
And, yet, the main variable isn't physical at all. It is above the neck.
"To begin with, as a starter, you have to be able to mentally hold your focus for three hours," said Bud Black, now San Diego's manager but the Angels' esteemed pitching coach during Escobar's latest switch.
"And you have to be able to function knowing that you're going to pitch every fifth day. A lot of guys can't handle that," Black added. "As a closer, you come to the park thinking you might not be in there today. All of a sudden, the phone rings and it's time. A lot of guys prefer that."
Wainwright, who never relieved anywhere until doing so on an as-needed basis for the Cardinals, returned to starting with a closer's mentality.
"The key to pitching for me is not saving anything," said Wainwright, who at 25 was young enough to get away with it. "I'm going to worry about the hitter at-bat that time, rather than three at-bats down the road. It's just not going to be successful for me. I tried it. If I start worrying about saving stuff, I'll find myself out in the fifth inning and then I won't have to worry about saving anything. I'm getting this hitter out right now. Whatever it takes to get him out right now. As soon as I can do it."
However, getting a feel for pacing is one of the most difficult adjustment for born-again starters.
"It's a different kind of pitching," Black said. "As a starter, you have to have the endurance to hold your stuff for a hundred-plus pitches ... and have the ability to recover from that workload every fifth day.
"The guys who go back into the rotation, foremost, have to have the repertoire to work their way through a lineup three, maybe four, times. You have to have a mixture of pitches to do that. In the bullpen, you condense your stuff, rely on maybe two pitches, sometimes only one if that pitch is dominant.
"But you can't throw 115 awesome split-fingered fastballs. As a closer, you throw five, six great splitters and the inning's over."
Five years ago, Dempster was a part of Florida's Fab Five: A rotation of A.J. Burnett, Josh Beckett, Brad Penny, Carl Pavano and him. Dealt to Cincinnati in mid-2002, he endured three years of elbow problems before moving on as a free agent to the Cubs, becoming their closer in 2005.
After 85 saves in 99 chances, he got the starter's bug -- and manager Lou Piniella's green light.
"He wants to start," Piniella said, "and we're going to let him compete."
"It's not going to be easy, going from 70 to 200 innings in a year," Dempster said. "But I feel good, my arm feels good. I know I'm going to have a good year.
"I learned a lot the past few years and I'm looking forward to starting. I'm not worried."
Dempster appears to have the economy part of it down pat: In his first two exhibition starts, he has logged five innings (allowing four hits and two runs) on only 35 pitches -- a phenomenal rate equal to a 63-pitch complete game.
"I just try and make pitches," he said, "and get quick out to keep the at-bats short."
Myers, who logged 50 wins in the rotation in four seasons before logging 21 saves as the Phillies' emergency closer last year, is proving starting is like riding a bicycle.
On Wednesday, he threw four shutout innings at Toronto in his first A-game start of the spring. Along with everyone else in Clearwater, Fla., he stays tuned to new closer Brad Lidge's condition (right knee surgery) to see whether he can remain a starter.
"I'm happy to do whatever," Myers deferred.
The most extreme role change was pulled off last season by Braden Looper, whose enlistment in St. Louis' rotation must have set back baseball 30 years.
The switch to starting has been made by younger pitchers after brief, experimental turns closing, or by veterans returning to the roles in which they had begun their careers.
Then there is Looper, who made 573 consecutive relief appearances -- 103 of them resulting in saves -- before deciding at 32 that he wanted to start. His personal revolution was a qualified success: He ranked second on the Cards with 12 wins (to Wainwright's 14) and went six-plus innings in 21 of 30 starts.
"The difference starts happening in Spring Training," Looper said. "The stretching out rather than throwing every other day. To me, I'm a starter now. It's been a great change that afforded me the luxury of working on things and improving on areas of my game that I have not been able to do.
"I'm a lot better pitcher today than I probably have been in my whole career. I probably had nastier stuff when I was younger, threw harder and had more sink. But I don't try to throw hard now. I just try to locate the ball and keep it down."
Nasty stuff in the short run, tempered stuff in the long. If you're looking for a road map from the bullpen to the rotation, that's as good as you'll get.