Lockstock
Sadly, you were advised correctly. The reason:
Your contribution rate was set as a percentage of your "actual rate of Military Salary" (basic pay) for the remainder of your Service.
The increased Death-in-Service Benefit that those contributions buy, within AFPS75, is a fixed multiple of your "Representative Rate of Pay", which is not the same thing and did not change when you became PA.
The future situation in AFPS05 might be different, but is actually irrelevant since you won't need this particular AVC in AFPS05.
Although, once started, contributions must continue until the end of Service, that does not necessarily prevent them being varied, or even Nil Rated, in my view, if the Defined Benefit that they were designed to buy is changed.
Off to lie down again now
Ginseng