[rlu]

So you can't "send as SMS" in between the iMessage having been sent but before it's been delivered?

[nickjackson]

Its a bit confusing, but in my experience, as soon as the message finishes sending, whether its been delivered or just hit apples server, the send as SMS option will disappear.

Sometimes the message will say its failed to deliver and you get the option back.

[fanf2]

You can double-tap it after sending to get the send-as-SMS option in a pop-up.