How to change iPhone texts back to iMessage?

Yesterday, I had my data usage turned off on my iPhone 4s, so naturally, I expected it to send text messages instead of iMessages to my friends with iPhones. However, when I turned my data back on, it still didn't change back from text message to iMessage. Both my and their iMessage is turned on, and we can both send iMessages to other people. How do I get it to change back?

Added (1). I've already tried turning it off and back on and it did not work.

Turn imsg off and back on to see if that fixes it.

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