From my experience, if you decide to try the step I described for someone who didn't manually turn off iMessage then your phone will essentially stay on SMS for that thread since it has determined that the phone you're trying to iMessage is "offline" so to speak.
If I turn my iPhone off and you try to send me an iMessage and your setting for "send as SMS" is switched off, it will never get delivered. Essentially the holding down and sending as a text message is the manual way. Once you do that and my phone continues to stay off then your thread will turn into a regular SMS thread until my phone is turned on again and connects to data. Same goes for if I'm in a dead data zone. It's the equivalent of turning off my phone or even putting my SIM card into a non-iMessage phone.