Others might have kept their old PTA EIN, but my gut tells me for you to get your own new EIN for your new PTO. That's form SS-4 from
www.irs.gov. Free. Easy. Step one.
You'll need to open a new bank account with the new EIN (unless maybe the bank will let you switch EINs???) and move your money to the new account.
With your own EIN, you then consider incorporating as a non-profit org in your state. That's also pretty easy and cheap.
Now you have at least 27 months from the day you are formed (that is, your incorporation date) to apply for your 501c3 status with the IRS. I say "at least" because the IRS says if your annual gross income is less than $5,000, you don't need to apply for 501c3. Once you hit that threshhold, or you expect to hit that threshhold, you should apply. If your old PTA consistently took in more than $5000 (notice, it's gross, not net income), then your new PTO probably will too, even if you're just getting started.
In the interim between your incorporation and your actual getting your 501c3, there may some consideration by the IRS as to the deductibility of donations made to your new group. You should call the IRS for clarification on that.
If you want advice in writing, consider getting the PTO Today Start Up Guide which includes all these steps and more. Come back to the message boards with your questions. There's lots of us out here who have been through the process.