They would have to be by default, since the Chang Hon style was banned in South Korea. Some" cheated" in training yet still gave out KKW certifikates because of their affection for the Chang Hon forms, while others never looked back, to this day.
I don't follow the logic. By "Kukkiwon instructors" I don't know which you mean out of:
- official instructors, with a paid salary or per-day-rate from the Kukkiwon
- instructors that are Kukkiwon-educated and licensed (I don't know when the Korean instructor course began)
- instructors that teach Kukkiwon-style Taekwondo
In case 1, what would "Chang Hon being banned from South Korea" have to do with them being former Chang Hon stylists? Most likely the paid instructors at Kukkiwon would be regular Kukkiwon stylists, from one of the major kwans, likely one of the "big three" (Chungdokwan, Jidokwan and Changmookwan).
In case 2, assuming the instructor course was in existence then, what makes you think the majority of attendees of that course were Chang Hon stylists (which is only early Ohdokwan) over the other 8 kwans (ignoring the administrative kwan)?
The most likely, case 3, again though is the same as case 2. Ohdokwan wasn't the largest kwan, so let's go with some estimate numbers - 60% of practitioners were from the big three kwans, and the remaining 40% were split amongst the other 6 (let's assume fairly equally). This means that early Ohdokwan (Chang Hon) had about 7% of Taekwondo practitioners. If they abandoned their Chang Hon roots, that's still not a "large portion".
Finally, I'd like to talk about "Chang Hon was banned in South Korea". Is there any proof of this? General Choi Hong-hi was exiled from South Korea, and as far as I know there was no official ITF presence in South Korea until 2004, but the Chang Hon style was banned? Who banned it? The Korean government banned a style of movement?
Ohdokwan definitely still has a presence in South Korea and has maintained that throughout, but they practice Kukkiwon style (the most famous current Ohdokwan practitioner is probably, IMHO, GM Hwang, In-sik - one of the Kukkiwon's senior instructors and chairman of the poomsae committee for KTA, KKW and WT).
So I'd like to reiterate Gnarlie's point - "Source?". Currently, I think your point is highly unlikely and currently illogical and unsubstantiated.