They did stop using the formula after the EH.
Remember the early 60s brought local competition from Ford and Chrysler. The old backwards letter system was well understood, and a broken code is no code at all. Its continued use would have put Holden at a competitive disadvantage, as the other manufacturers would know when Holden intended to implement something substantial. (For example the HR was the first Holden to have seat-belts as standard IIRC, and the first to have a 4-speed. Had the old system remained, the HR would have been the EE, and a competitor would know Holden intended to introduced these things in 1966 if they heard the relevant model code "on the grapevine".)
The HD is named for the Managing Director of the time, David Hegland. Throughout the "H" series they used random letter codes, and then with the the "V" series the codes were alphabetical, with random gaps. (Although you probably could have guessed the VC Commodore would not be followed by the VD
)
cheers
RET