To reduce to absurdity the argument being advanced against PhillyT; someone fields count-as DGKs converted from Thunderwolves. They come on a round base.
If it is okay to convert (now) the IoB HE gryphon to be an Imperial gryphon and keep it on the base it came on, why not Thunderwolves?
This is the same argument - unless we want to introduce additional details to the exceptions, making a case for cross-game changes.
I think if you convert a model from a different line you should base it on the appropriate base when you make the conversion. Then, it can be grandfathered in.
So, an IoB gryphon made six months ago into an ImpGryph would be 50mm square, made now would be 50x100mm.
Otherwise, it is a bit of a cheat, no?