Well for me personally I consider "canon" to be ultimately the intent of the creators rather than what appears on screen (for the most part). Otherwise all the retcons Star Trek has would be difficult to explain. But I'm happy to believe that whatever Mike Okuka and co. say is canon is canon. But it's tricky, I will happily believe that the Zhukov is NCC-26136 and not 62136 even though the model said so (and misspelled the name), I'll happy take that as an error as it wasn't seen anywhere onscreen and then later was changed. I also believe that the Hood was always ncc-42296 and the 2541 never happened and neither was there a nebula class Melbourne despite the model being made and destroyed. That said the "Brittain" is a little harder to explain due to the massive closeup of the name it's hard for me to say the ship was supposed to be "Brattain" when it's staring you right in the face. So yeah it's a case by case basis but getting back to the Excalibur, I am pretty comfortable that the intent was that the ship was to be identical to the Zhukov and Yamaguchi. Whether the Horatio was also a variant is still open to interpretation as it's registry number is in the 10,000's so it could have been like the C but I'm still leaning to the idea that it had been upgraded just like the Zhukov/Yamaguchi.
The changes to the Ambassador class could be likened to the changes the Excelsior received between Star Trek III and VI also the upgrade of the Constitution class as well. So yeah I'm starting to feel that they were probably all subsequently altered some time before TNG and the destruction of the C.
That said, this guy has another argument and it is convincing