There is no need to replace your modem if you are going to upgrade your internet speed. Your connection is actually provisioned in the AT&T server and the purpose of your modem is just to deliver the said speed. Basically, all DSL modems should be capable in handling any connection speed. However, there are certain factors that could possibly affect the efficiency or performance of your modem such as outdated firmware version, mechanical and electrical drawback, and some interference that surrounds it.
You may just ask our Sales Team if they could make some arrangements for you to get a new modem when you upgrade your speed. You can call them at 888-321-2375 and just say "Sales" at the prompt. Their hour of operations are Monday - Friday, 8:00 am - 7:00 pm and Saturday 8:00 am - 5:00 pm.
*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.