If you backdate an Item Valuation report, there may have been transactions that have occured for an item between that back date and today. The report takes this into consideration and adjusts the Qty on Hand and Total Cost for the item, to calculate what the Qty and Cost would have been on that older date. In addition, the Avg Unit Cost is calculated as the Total Cost divided by the Qty on Hand.

If you see Costs and Avg Cost change for a backdated report as compared to a report as of Today, there has been at least one transaction between the report date and today.

Note that if any Transactions for an item have been purged out of IC History, or if processes such as Print/Purge the OE Invoice Audit List have not written the transactions to History, then any missing transactions will pretty much guarantee that a backdated Item Valuation report will not have the correct qty and/or cost.

Also if OrderEntry is used, if Day End processing has not been run then this also would give incorrect values for the Item Valuation report.

I would be very surprised to find that changing the Category would affect the costing information, as it only affects GL accounts for costs and revenue.

If you changed the unit of measure on the item, that has a potential of affecting the report, as Historical Transactions may use the previous units. Perhaps take a look at the Stock Card report, which will detail all of the historical transactions for an item between particular dates. The logic behind this report is the same as for a backdated Item Valuation.