Gar had been known as "Beast Boy." This lasted through the Teen
Titans West era. Marv Wolfman and George Perez decided to change
Gar's ID to Changeling. Prior to Gar's name change, there had
been a character in Marvel comics named "Changeling" (he had died while
impersonating Professor X).
So why the change back to Beast Boy?
There was a period of time when Gar wasn't being used in any comics
(sad but true). In that time, an RPG company created a fantasy
game called "Changeling: the Dreaming."
When Marvel decided to resurrect their "Changeling" character, DC had
taken over the name and, rather than have any kinds of legal battles,
they renamed their character "Morph." My suspicion is that, when
DC decided to bring Gar back to the forefront, his name was
taken. Rather than undertake an extensive legal battle, they
probably decided it would be easier to return him to his old name of
This was later explained in one of the comics as having been the result
of his return to the west coast, where people knew him better as Beast
Boy from his earlier days.
In my opinion, DC could have done better - either they could have
advanced Gar to his Kingdom Come name of "Menagerie" or come up with
something new (in Brazil, Gar's name is "Mutano").