Hi,

In Odoo 15, changing the valuation method from AVCO to FIFO after transactions and valuations have already been processed requires careful handling to avoid data inconsistencies.

Directly switching the product category’s valuation method to FIFO will not retroactively recalculate past valuations; it only affects future transactions.

To ensure accuracy, complete all July transactions under AVCO and close that accounting period to prevent further edits.

From August 1st, create a new inventory adjustment reflecting the current on-hand quantities and values as of July 31st. This adjustment will serve as the opening balance for FIFO.

Before applying the change, record the stock valuation report under AVCO and reconcile it with accounting to preserve a proper audit trail.

Update the product category valuation method to FIFO and ensure “Automated Valuation” remains active so future movements use FIFO automatically.

Document the change internally (and ideally through a journal entry note) to indicate that FIFO is applied prospectively from August onward and that prior periods remain under AVCO.

For further details you can refer this blog: https://www.cybrosys.com/blog/what-is-inventory-valuation-with-the-odoo-15-inventory-module

Hope it helps.