Ok i will try to run through the steps i used to make this work.
Note this is the only way i could get all the settings including IP address to stay the same.
1) Make backup of settings on A1408. Making a note (write down) all info subnet, IP address Domain name etc..
2) Unplug power from A1408
3) plug in power only into the A1521
4) find A1521 in Airport utility
5) Run through the set up but do not plug in the ethernet in cable when it prompts you.
6) after set up is done import the backup file onto the A1521.let that finish.
7) Now edit the A1521 and set to static IP address then manually input the DSN, IP address etc.. u want and the domain name. Save this.
8) after restart plug in the ethernet cable in. and you should be done.
This is the only way i could get the Airport to keep all the settings including IP address. Which is important to me as i have several IP cams hooked into this network that use the one IP address.