Remove panel from inside of cabinet use existing panel as a splice box remove all breakers and guts use seal tight four main feed and seal tight for circuits branch off to new 125 amp Inch and a quarter seal tight it’s gonna be needed inch and a quarter seal tight connectors as well. If seal tight does not fit in location I suggest to use 1 1/4 LR and EMT. There will be a diagnostic done when technicians come back to install panel if there’s any broken wires or circuits that need to be relocated or repair there will be an additional charge.