Colleagues with my former teacher

I had a teacher in kindergarten (as did everyone!) and I had her again in sixth grade.  When I was substitute teaching, it was strange working with her as oppose to having her teach me.  I even substitute taught in her class for her one day.  Now she is working with the teachers in my building.  Today, she came to my lab to show me the program the teachers and students are doing.  I'm sure it is strange for her too.  She was talking to me like an adult that knows about teaching instead of as a kindergarten student like she did 20 years ago.  I still cannot bring myself to call her by her first name in person.  When I talk to other teachers about her, I say her first name to them.  Even that's challenging for me.  She has told me it's okay to call her by her first name.  Considering she is my colleague now, I really need to get used to calling her by her first name instead of always saying Mrs. (name).  Wouldn't you find it difficult to call your former teacher by his or her first name?