Hi,
The new tax procedure is a copy of existing tax procedure only and you are making changes there. It wont create any other impact. SAP recommends this approach when you are changing the pricing procedure in productive environment.
If you are directly changing the existing procedure, it may create some inconsistencies explained in the note mentioned in my previous comment. If you can manually check and correct these kind of inconsistencies (which may occur only for existing open Po's only - not for new PO's created after the procedure change), you can proceed with the change..
Regards,
AKPT