You turn off "Improve the model for everyone" in Data Controls. What happens to the conversations already in your history?

The principle — this control governs what your conversations are used for, not whether they exist.

Why the key is correct — OpenAI states that after turning the setting off, your conversations will still appear in your chat history but will not be used to train ChatGPT. Two separate things are being decided, and this switch decides only one of them.

Why the others are wrong — nothing is deleted, because deletion is an action you take yourself. Your chats do not become read-only; you can continue any of them as before. And they are not reclassified as temporary chats, since temporary is a choice made per conversation before it starts.

Remember this — the setting is about OpenAI's models. If your goal is that a conversation is not stored either, that is temporary chat, and if your goal is removing what already exists, that is deletion. Two other useful facts: the change applies to your entire account across every device, and you can turn it back on at any time with no restrictions.

Sources — OpenAI's Data Controls FAQ.