The Florida Marlins retired No. 5 on April 5, 1993, in honor of the late Carl Barger, the club's first president and chief operating officer.
Barger passed away on Dec. 9, 1992 at the Winter Meetings.
Since Barger's favorite all-time player was Joe DiMaggio, the Marlins decided to retire the No. 5 in their first president's memory.
Now the Miami Marlins, the franchise has relocated to a new stadium in the Little Havana section of Miami. The team has a different logo and uniforms from what they wore in their inaugural 1993 season.
On Twitter, Morrison thanked Marlins owner Jeffrey Loria for allowing him to switch from No. 20 to No. 5.
"I understand what an honor it is to have a number unretired," Morrison said via Twitter. "I want to thank Mr. Loria and the Marlins for making it happen.
"I also promise to wear the number with great pride and continue to honor Mr. Barger and his family."
The No. 5 has special meaning to Morrison.
Morrison's father, Tom, encouraged his son to model his game after Hall of Famer George Brett, who wore No. 5 during his playing career with the Kansas City Royals.
Tom Morrison passed away from lung cancer in December 2010, after his son's rookie season.
The No. 5 had been the only retired number in Marlins' history, other than No. 42, which was retired throughout baseball in honor of Jackie Robinson.
When Morrison asked the Marlins if he could switch to No. 5, the club went to the league offices to get permission. Once that was granted, the club opted to allow Morrison's request, and will honor Barger with a plaque in the main concourse of their new ballpark.
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro. This story was not subject to the approval of Major League Baseball or its clubs.