Hi,
For MM documents; as the Taxes are always calculated in real time in PO; SAP has already issued note on the same for migration.
However, for SD documents (Open SO) there is no need for cut over. There is a functionality of redetermining the pricing procedure in billing document (Assign document pricing procedure in billing document). This will ensure that whenever open SO (in TAXINJ) is processed in TAXINN; system will redetermine the pricing procedure and post the same correctly.
Trust this clarifies.
Thanks.