Getting from Roma Tiburtina bus station to Roma Termini train station.

I will be traveling from Siena to Rome and will get into Roma Tiburtina station at 8:55AM on Saturday Nov 8. I need to get over to the Roma Termini station for my next connection to Bari. I understand there is a subway you can take to the Roma Termini station that takes about 7 minutes. I think I have enough time to do this as my Rome-Bari train leaves at 10AM. If I buy a ticket on line for a certain time say 9:14 and miss it, does anyone know if I can use that ticket on the next available subway train? Thanks.