That's even weirder, since the Moo Duk Kwan started out with the Pinan when it was Tang Soo Do, changed to the Palgwae with the unification, then went back to the Pinan forms when part of the MDK left the unification and went back to Tang Soo Do. That was eventually changed to Soo Bahk Do, but I believe still uses the same forms. The Chang Hon forms are no part of that. So the lineage of that school is probably somewhat sketchy.
I do not believe there is a system for grandfathering ranks. There are, however, plenty of people with KKW rank who will sign the papers to turn your non-KKW rank into KKW rank. The KKW does not sanction this, but there is no enforcement and never will be.
There are a few problems with this.
When you sign the KKW promotion form, you are attesting that the person you are recommending meets the KKW requirements. So if they don't know the Taegeuk forms, you're lying. Happens all the time, but I have issues with the dishonesty involved.
When this sort of conversion/assimilation occurs, there may not ever be an actual physical meeting between the two people. Just mail them a check.
The school you mention needs someone with KKW rank of 4th Dan or higher to sign off on promotions. They may have earned that rank, or it may have been a transfer. But in either case, the fact that they're offering KKW rank without teaching KKW curriculum calls their integrity into question. Personally, I think integrity is important.
Our students can get Moo Duk Kwan and/or KKW certification. But if they want KKW rank, they must know the KKW curriculum.