Coordinate with the inspector and power company for a time to pull meter and get inspection on new panel.
Once this is done be prepared to remove interior of existing panel to connect new circuits ran to the new panel for the inspector and then have the inspector to release the new service to the utility. The existing panel enclosure becomes a junction box so be sure to secure the hinged door to the cover.
If you prepare properly then it can all happen in less than an hour.
You can leave the old panel but there can’t be any three wire dryers or ranges connected to the existing panel and the conductors from the new panel to the existing panel will have to be replaced with four conductor cable.
The bonding will have to be removed along with any electrodes installed in the existing panel.