Do you have an end of year event like an ice cream social? You could create a banner to thanks the families that are moving on. We did this and had the banner in our cafeteria for like a week and asked all the kids to sign it. Our banner said: Farewell to all departing families. Thanks for all you did to make our school great!
As for welcoming new members and asking for input I agree with mommytlc - a newsletter is a good place to do it and keep it short and sweet so it gets read. Also could could put it on your website.
I was just elected PTO president at our elementry school.. I have worked on the pto for several years and would like to say goodby to our wonderful members whom are leaving us due to their children moving to secondary schools. I would also like to welcome our new board memebers and invite them and staff/faculty and parents to offer ideas, concerns and advice to our new board.
How would I do this all in One Letter??
As the new president I would love to see many changes in our school, especially more teacher involvement!