If you enter 'quantity shipped on orders' with the original order, then tax is calculated in the background but not actually charged until you invoice. If all you did to invoice that March order was to edit it and post it as an invoice, then I don't believe tax would get automatically recalculated in this case.
If you don't put quantity shipped on orders, then everything that Steve wrote applies.
You may have to temporarily change an order to force a recalculation of tax. This could be as simple as changing the customer tax status to something else and then changing it back (you don't have to save anythng between the changes). If you have any other outstanding orders entered prior to the tax rate change, try this and see if you get different results.
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