I'm planning to upgrade my existing 100 A 20 slot main panel to a 200 A 40/40 panel with a new 200 A service. Is it possible to install the 200 A panel next to the 100 A panel and feed the 100 A panel from the new 200 A panel. I was considering doing this to avoid having to redo and possibly extend the existing wiring already in the 100 A panel.
This would also allow me to run the new service cable, meter box, and panel without having to shut power off to the house. Then after it is all inspected I can have the power company disconnect the old service and connect the new service after the two panels were connected together.

Yes, no problem. Just need to be sure to run a insulated neutral to the insulated neutral bar, do not bond to the metal can, and run an #8 equipment ground, can be bare or green, to a separate equipment ground bar in the 100 amp panel.

Excellent idea, working on new keeping the old in use. Does not happen too often. That is done whenever possible.