Cancelling And Re-booking A Flight. Why Didn't I Get A New Trip Id #?

Look I don't understand your polices, but I booked a trip from Melbourne Florida to Pittsburgh Pennsylvania on 2/29/12, I had to cancel that trip, but yesterday I rebooked this similar flight from Melbourne Florida to Pittsburgh, Pennsylvania for these dates: Departing Melbourne Florida on April 27, 2012 to Pittsburgh Pennsylvania, return flight on May 1, 2012.  Now I did not get a new "Trip ID #", so therefore I am writing to C/S to get a definitive answer about this flight.  First, the C/S person told me that there would be  only @ $90 charge for the rebooking because the new flight on April 27 - May 1, 2012.  I did not receive a NEW (TRIP ID NUMBER). Please clarify if it is ok with the airlines that I use the old trip ID#.  Flight # out of Melbourne Delta #2005 (need and asle seat for health reasons)_Atlanta Flight # 1295 arriving Pgh. @ 11:19 am. Return back to Melbourne May 1, 2012 Delta # 240 departing at 12:00 noon arriving in Atlanta @ 1:43, Delta Flight #2195. Arriving Melbourne, Florida @ 4:57 pm.  So why have I not received a new Trip ID #.  What I have received from C/S doesn't look like anything I have ever received from Delta Airlines.  I DO NOT want to encounter an issue when I go to use these tickets.  I want to know I am confirmed on these flights with assigned seats. Thank you for your immediate attention to this request. Sandra M. Hoffmann ([email protected]) Reference Trip ID # 320597746252 cancelled and re-booked.