Wayne,
is this version 4.4 or newer? if so the report should be showing the ending valuation not just the change. As far as I know it is the only way to get that valuation for anything prior to the current valuation. Prior to 4.4 I just told people that it was a waste of time to even run the report. I still don't have 100% faith in the accuracy of the report in 4.4 and newer. I sometimes see weird things such as zero on hand with a dollar amount in the ""inventory value"" column. (sometimes even zero on hand and a negative amount in the value column). But for the most part I think it will give a very close valuation for a prior period.
Because I don't yet fully trust that report, I recommend clients run the regular valuation at month end after all ROG and SO invoicing is completed and prior to anything posted to the next month; however, this is rarely possible. My next recommendation is to run the Inventory Valuation Report by period as soon as possible after the ROG and SO invoice is complete for a given month. My gut instinct is that the farther back you try to go the more likely it could be incorrect.
I think you need to make sure all the settings in Common Entry for Years to Retain Item History is set long enough to cover the period you are trying to get a valuation for .