How to disable iMessage and switch to SMS for everyone

Trust Wallet has just posted that there is a $2M iMessage zero-day hack being offered on the dark web.

So until we know it is safe for sure, how do we turn of iMessage and just use text?

When I turn iMessage & Facetime off (including turning off wifi access on my phone, and ensuring MMS is set to on) my iphone 6s refuses to send a message to a regular recipient who is also on an iphone because iMessage is set to off.

Does it just take a while to deregister my number from Apple's iMessage service?

There must be some way to get my iphone to bypass the need to be using iMessage?

[Re-Titled by Moderator]