The only sure way to change IP is a port swap requiring a tech service call. The port needs to test bad or will not be approved for swap. A customer requested port swap is a Dispatch On Demand (DOD) with $149 billable service call.
Sometimes IP address will change after gateway swap, or software update is pushed out, but often does not.
The other option is cancel service, pay any ETF required ($15 per month for each month remaining on contract), return equipment. Sign up as new customer account, new install date, different gateway. Generally will have several days of no service between cancel and the new install. Subject to new customer $99 installation charge.
*I am an AT&T employee, and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.