I don't think it's an unnecessary retcon, I think it makes perfect sense. In G1 Swoop transformed into a jet on Cybertron as part of the Autobot Elite Flying Corps (as seen in the 1987 Annual story "What's In A Name?").
Swoop's Cybertronian alt mode
Swoop's Cybertronian robot mode
So while it wasn't until Dreamwave's War Within that we saw the Cybertronian alt modes for Grimlock, Slag, Snarl and Sludge, it was already established way back in 1987 that the Dinobots did have Cybertronian modes different from their dino modes.
And I personally think it's necessary because isn't the entire point of giving the Transformers Cybertronian alt modes part of enforcing that element in TF lore that the Transformers transformed into non-Earthen alt modes prior to their contact on Earth?? It makes no sense for them to transform into Earth modes if they're on Cybertron millions of years before even making contact with Earth.
Are we supposed to believe that by some massive coincidence Grimlock's pre-Earth Cybertronian mode just happens to be the same with what he will transform on Earth? The game designers went to extensive efforts to give every Transformer a different (yet recognisable) appearance in War For Cybertron... seems strange that they would make an exception just for Grimlock.
Which would you honestly have preferred seeing in War For Cybertron:Originally Posted by Hursticon
+ Bumblebee as a Cybertronian compact car, or as a Volkswagen Beetle
+ Optimus Prime as a Cybertronian truck, or as a Freightliner short-nose cab truck
+ Megatron as a hover tank, or as a Walther P38
+ The Seekers as Cybertronian "tetra jets" or as F-15 Eagles
+ Soundwave as a armoured vehicle or as a portable radio cassette player
+ Ironhide as an armoured vehicle or as a Nissan Cherry vanette
+ Warpath as a BLAM! Cybertronian tank, KA-POW!, or as a ZOWEE! M551A1 Sheridan ARAAV tank, KA-BLOOEY! (WFC Warpath remains my most lovable character in that game! BOOM!)
...etc.?
Optimus Prime and Bumblebee look Cybertronian to me. <shrug>Originally Posted by Hursticon
But if this story takes place after Earth contact, then I take back any criticism I've made about Grimlock's dino mode. It's only if the story occurs prior to contact with Earth that seem illogical to me... unless they're out to prove that there WAS organic life on Cybertron before the Transformers came about! But even then... the odds of said organic life being identical to Earth? (meh why not... they did find a fossilised bat in the core of Cybertron in Beast Machines)