From what you wrote, you do not actually want to change the vectorizer, in both your schemas you state it as text2vec-openai. So the embedding model will stay the same and with that, you can keep your collection with the precalculated embeddings of that type.
The generative part is different from the vectorizer, it is used in RAG to come up with a response suitable returning to a user query at runtime (while embeddings are calculated during data ingestion). So it makes sense that Weaviate does not have a problem with the user changing that setting lateron. Hope that clarifies things - if not, maybe check out the documentation here to read more on the topic: Generative AI | Weaviate