Among a great number of other things, Section 408.2 requires panelboards to also comply with Article 312.
Section 312.8 requires enclosures for OC devices to provide adequate space for conductors feeding through or tapping off by filling the wiring space at any cross section to no more than 40% fill for conductors, and the conductors, splices and taps no more than 75% of the cross sectional area of that space.
If you have the room, then the existing panel can be used to splice extensions into the new panel.
