Okay, you will need the old base for this process.
Unplug the new base for now.
Plug in power to the old base, so that we can deregister the old handsets that are linked to it.:
Press and hold FIND HANDSET on the telephone base for about 10 seconds (until the IN USE light on the telephone base turns on and starts to flash), then release FIND HANDSET. Press and release FIND HANDSET again.
You must press FIND HANDSET while the IN USE light is still flashing.
The IN USE light flashes for approximately seven seconds.
All system handsets show CONNECTING... The deregistration process takes about 10 seconds to complete.
All handsets show NOT REGISTERED when the deregistration completes.
Once they are deregistered, unplug power to the old base, plug in the new one, and I'll guide you through registering to it.