Hades,
Bill Slavicksek introduced the confusion when he wrote the revised box set. According to Slavicksek, the champions (who were not dragons) spontaneously decided to turn Borys into the dragon after defeating Rajaat, despite there being no precedent for it (again according to Slavicksek). Doesn’t make sense? I don’t think so either.
What does make sense it that the champions (who WERE dragons) accelerated Borys’ transformation into a fully grown dragon (probably Borys because he was furthest along the transformation already).
If you believe Slavicksek, Borys subjected himself to an experimental psi-magical spell of uncertain outcome.
Why did Slavicksek do this? Personally I think he just messed up and didn’t do proper research. Lynn Abbey noticed all the canon problems (again, introduced by Slavicksek!) and did her best to reconcile them.