In an interview with Famitsu Weekly, Tetsuya Nomura, director of the Kingdom Hearts series, was asked questions concerning the HD collection announced for PlayStation 4, Kingdom Hearts HD 2.8 Final Chapter Prologue. One topic in particular was brought up about Kingdom Hearts 0.2 Birth by Sleep -A Fragmentary Passage-. In the discussion, Nomura explains the distinction between "0.5" and "0.2". "0.5" was supposed to be a whole new volume, as hinted at in the Kingdom Hearts Birth by Sleep: Final Mix ending of the secret episode. The reason why "0.5" was bumped down to "0.2" was because Kingdom Hearts III is still a priority and did not want to affect development for it, however Nomura wanted to give fans a taste of Kingdom Hearts III with it's new technology, which is used for Kingdom Hearts 0.2 Birth by Sleep -A Fragmentary Passage-.
You can read the quote below from the Famitsu Weekly interview thanks to KH13 News Team member soraspromise.
I see! Having to shake the time axis of the series to make new additions. The 0.2 in “KH0.2 BBS” especially had to have a deliberate significance.
Nomura: In the secret episode of “KH BBS Final Mix”, there was a display of “0.5”. “0.5” was originally intended to be a full volume as a sequel to “KH BBS”. But because “KHIII” is still the top priority, “0.1” took one step forward to “0.2”.