The first half of Wild Force is Viktor Adler, having consumed the remains of Master Org's previous host, possessing his memories while still in a human form (thus necessitating gear to replicate an Org body). He thinks of himself "as" Adler, even while remembering the identity of Master Org. Upon Mandilok's murder of Adler, Master Org's spirit takes full command of the body. "Viktor Adler with Master Org's memories" turns into "Master Org with Viktor Adler's memories."
Master Org remembers being Adler pretending to be Master Org, with Master Org's memories. Hence his speech to Shayla that, even if he wanted to turn back now, it's too late anyway. Even if their identities are inexorably wound together, he can't just go back to being Adler. This is "Human Org." Once he consumes the Org Heart and reverts to a wholly-Org form, the matter is settled.
As for why Master Org's original humanoid form three thousand years ago resembled Viktor Adler, while his fully mutated form in the past looks totally different than Final Form Master Org in the finale? Dramatic convenience.