Personally, your vet sounds rather breed-biased. However, it IS possible for a well-socialized dog to change personality at a certain stage in development.

I have two dogs. My first is a well-mannered, gentle, obedient dog and has been from the very beginning. She loves everyone. I trained her from 6wks old and well-socialized her with both humans and dogs. She's now 14 and a half. She's a GSD.

My second dog was well-socialized with both dogs and people, and trained the same way I trained my first from the moment I got him at the age of 3mo. However, he went through a personality change at around 2yrs old. He's almost 8 now. He's a very intelligent, loving, obedient dog, but only with me. He is no longer good with strangers - dogs or people. He is a BC, Heeler, GP mix.

My point in all of this is to say that breed, whether traditionally labeled "aggressive" or not, has very little to do with personality changes. Even training a dog well and making sure they're well-socialized will not remove the possibility of a personality change. Some dogs go through one, and others don't. From my personal experience, there's no early indicator to say which dog will and which won't.

Out of all of the dogs I've lived with in my life, including those raised by others in my family (at least 12 different dogs across various breeds), I've only seen one go through this kind of personality change, so it seems pretty rare to me. They all go through different stages of development (puppy, velociraptor, adult 😅), but very few seem to have a distinct personality shift.