How to keep messages after deactivating imessage? I disabled imesseage (and removed the addresses) on my iphone 4. Afterwards, some old messages to some friends disappeared from my phone. I don't understand why.

I restored imessage and the addresses, and now the old messages are back. How can I disable imessage without losing the messages?
In the morning, reactivating imessage, adding back the numbers, then disabling it removing the numbers, fixed it. I still have all my old messages.