How do you transition from two country based to a single tax zone based Tax Regime?

Summary:

We wish to move from two country based tax regimes to a single zone based regime.

We would like advice on how to transition the two regimes to a single regime given there will be in flight transactions. 

Content (required):

We suspect there needs to be a cutover date specified in the Configuration Owner Tax Options, otherwise either the old country based or the new Tax Zone based Regimes may not calculate tax if there is any overlap.

Version (include the version you are using, if applicable):

23B (11.13.23.04.0)

Code Snippet (add any code snippets that support your topic, if applicable):