Do all plans in the 1.0 live system need to be mapped to a 2.0 item price id i.e. even archived ones?

There might be subscriptions and invoices records linked to the plans and addons. So it is necessary to move all the plans and addons to Latest Product Catalog, including the archived ones.

However, if there are no subscriptions attached to the plans and addons that you do not want to move to Latest Product Catalog, you can delete them right away in Legacy Product Catalog.

To handle archived plans in Latest Product Catalog

Going forward, you could make use of grandfathering feature of Plans which is explained here, since you are creating new Plans only to introduce price change. This is supported in Legacy Product Catalog as well