Release picking up old tax code

Where do I change the setup of the tax code in a BPA. We have a BPA out of wheich if I create a release it generates an old tax code of 5%. I have checked the BPA and the 'Manage BPA' menu is greyed in 'Purchase Order' screen. If I open the BPA under PO Summary screen I can navigate to tax screen, however, there is no tax line. When I create a release against that BPA it picks up old tax. I end dated the old tax in a test instance and now it does not pick any tax.