J2D, I saw the BGG post you are referencing. There is another
post by the game's designer explaining why the change:
"Counter changes: After much player input, we changed the defensive values for AT guns from blue to red. This defensive value was also lowered, so that medium tanks still had roughly the same hit probability. But now these AT guns are more susceptible to MG fire. Players convinced us that this was more historically accurate and we agreed."
If we are voting, I would vote to change all the DVs to Red as per the third edition of the rules (the ones found in AtB Second Edition). Logically it does make sense the ATG shields would only protect against small arms fire, or red attack in CoH terms.
Now whether Eric can implement that (sounds pretty easy on the service), is another thing.