Hi Ivan,
Please test all of the following in your test database first (see my previous answer):
As far as I can tell, it should be possible. You will need to use the Inventory Revaluation document (Inventory > Inventory Transactions > Inventory Revaluation).
This functionality lets you determine the quantity and warehouse for which you want to change the value and you can change the Posting and Document dates to a date in the past.
For the date in the past, you may need to check and/or change that you are allowed to do so: Administration > System Initialization > Document Settings > Block documents with earlier Posting Date.
To determine the correct quantities to reevaluate you would do best to use the Inventory Audit Report (Inventory > Inventory Reports > Inventory Audit Report).
Regards,
Johan
