From one box to the other

Before getting estimates and electricians out here i'd like to get a heads up on this as much as possible. I would like to move from an inside breaker box (FPE) to an outside breaker box (cutler-hammer) 12 breakers?

Other information that might be helpful:

The distance back to back on the boxes is @ 12-17"

I briefly explained this idea with an electrician ,and he said it sounds like one box is a sub panel or "piggy back" as he called it. The inside box is 100amp,and the outside box 125amp. He also said if the switch,from one box to the other is possible the inside (FPE), box would become a junction box.

Thanks in advance to all!!

If you install a panel outside that is the service point and requires a main breaker as well as the individual circuit breakers.

The Grounding Electrode Conductor must be run to the service point without a separable splice.

If you need only the 12 circuits/spaces provided by the outside panel, then there is no need for a subpanel inside. If you need, or will need, more than the 12 circuits/spaces (Each 240 Volt circuit requires two spaces) then you might need a subpanel inside.

The existing inside panel could be converted to a junction box to permit extending the existing wiring from the current box to the outside box. If the insides are removed there will be plenty of space for the wiring connections.

I would run a few PVC conduits from inside to outside with THHN/THWN wire from the inside to the outside. If you try to do it all in one conduit you will have serious derating issues and will have to use larger wire.

You will minimize the derating issues if you run Multiwire Branch Circuits from the new box to the junction in the old panel which becomes a junction box. The MWBC need not go any farther than the junction box where they will divide off to the existing circuits.