Can I use my old service panel as a sub-panel when I install a new one?
Yes, you can do that. You will need to remove the nuetral->ground bonding screw from the subpanel. The new panel should have its neutral to ground bonded (unless that is already done at the power meter). You'll need to install a 100 A circuit breaker in the new panel (or higher, if the subpanel and wire are rated for the higher current) to feed the subpanel. You'll also need to make sure that you have a separate neutral and ground wire running from the main to the subpanel.