Can´t Update Tax Code in PO Line once PO is approved

Hello Experts!

I´m having a little problem.

I made a PO, insert the tax code and approved it.

But I want to change the tax code and the the tax field is blocked.

I entered a new PO line and with that line it´s everything ok. But with the old one it´s impossible to change the Tax (the field is blocked).

I´m sending a picture in annex.

Kind Regards

Diogo