Julier has it pretty much right. The only other issue you may want to look out for would be legal -- if you're incorporated and/or 501c3 and/or have bank accounts in your group's name, then you'll need to do some paperwork to make your name change official. Definitely do-able.
PTO is a generic term for a parent teacher group,so are the others you've mentioned. PTA is a state and national organization- you have to be dues paying members to call yourselves a PTA. Our group is none of the above, we are called an Elementary Schools Association. Our group covers the two grade schools (a K-2 and 3-5) in the town.
I can't imagine what trouble you'd be in by calling yourselves a PTO. Of course, if in your by laws you are called something else, you might want to make an amendment to change your name.
Hi all. Got a quick question for ya... It turns out that our "PTO" is not even a pto. We are called The Capron Grade School Club. For as long as I've been involved we have been using the Capron PTO name and have found out that we could get in trouble for this. Can we change our name to PTO, PTSO, OR PTSA? What do we need to do to complete this? Thanks for your help in advance.