Looking to up grade my electrical service to 200 amps. I'm going to hire and electrician to do the main hookup to the outside disconnect and the new 200 amp panel. I'm looking to do the inside wiring myself. The new panel is going to be in a different spot than the old one (about 20 feet away). Can I (temporarily) treat the old panel as a subpanel while I bring over the main lighting circuits one by one?

Probably. Just make it part of the Statement of work. Lic. Electrician install 200 A disconnect and main panel and temporarily wire the existing main panel as a 100 A sub panel with a 100 A breaker. Inspect.

When work is complete in moving the circuits arrange a final inspection.

100A will be the largest breaker you will be able to get.

The neutrals and grounds will have to be separated in the existing panel and the bond removed. You may actually have to purchase a ground bar kit to do so.

You will have some extra costs involved: Ground bar kit, 100 A breaker, wire .

You can probably have the electrician pull the first permit and you pull the second.

Best bet is talk with your electrician and the AHJ (Authority Having Jurisdiction).

You need to check with the utility company to see if they will allow the panel to be moved to another location. Also, the Main panel must be as close as possible to the entry point of the service.