How do I change the budget of a line on an approved purchase order?

Open the purchase order you want to update and click Edit. Any user allowed to edit the PO can change a line's budget.
On the line, open the Budget dropdown and pick the new budget, then save the PO.
If the new budget changes who must approve (the approval route), the PO automatically goes back for approval along the new route, in full.
If the approval route is unchanged, the PO follows your normal edit/re-approval settings, just like any other edit.
The timeline and the approvers' audit note record who moved the line, the old and new budget, and whether the change re-ran approval.

Budgets are reconciled on save — the old budget is released and the new one charged, so overrun rules still apply.

An advance payment tied to the line follows the line to its new budget so the payment stays correct.

Move a line's budget on an approved, delivered or paid PO and see it re-routed for approval when needed.