If the new location is in a different wire center, Sonic.net's policies won't allow you to have the old number at the new residence. However, there is a way to get this done, but it will take a few days and possibly a few $.
1. Establish Fusion service at the new residence, with a number assigned by Sonic.net.
2. When you're ready to relinquish the line at the old residence, go get a cheap "disposable" pre-paid cell, such as from Virgin Mobile, Net10, TracFone. You can often find the phones for < $20 in stores, or maybe even cheaper on Craigslist. Don't worry about the features, as you won't be keeping it long. Buy the minimum number of minutes required to establish service.
3. When you establish service on the cell, tell the carrier to arrange to have your AT&T number ported to the cell. That should take a couple of hours.
4. Once the cell is receiving incoming calls to the old number, register for a Google Voice number (
https://www.google.com/voice). When you get to the point where you need to select a number, tell Google Voice you want to port your mobile number to Google. There's a one-time charge of $20 to do this.
5. The mobile-to-Google Voice porting process takes about 24 hours. While waiting for this, program Google Voice to forward all calls to your new Fusion number.
6. Once the port is completed, calls to your old AT&T number will ring on your new Fusion line.
7. Google Voice has a bunch of other features you probably won't want to use, such as call screening. Just turn them off.