Medgirl2001, If I were you I call retention and ask them for 20 dollars messaging plan. It is 20 dollars for all lines combined. They don't give to you, hang up and call again. 550 plan has no SMS plan, but they can also do it for you. 550 has only 3 lines on the plan and they cannot change it, alrast for me, so I stopped on 700.
Thanks to you and dancefreak and everyone for talking about this. Retention changed our family messaging plan to the $20 one without any trouble. I'm still not sure about moving to fewer minutes than the 700 plan, more because I'm not sure if we might want to be able to have four lines in the future, but changing the messaging is a nice savings.