There are a couple of ways of looking at what you are shooting for here.
I would say starting with the PO: you should update cost upon receiving the PO, before committing it. Then using the option under SO Manager> File> Config> Inventory tab> Cost update = Weighted Average.
But - this method works only if you are really relying on POs to move new inv into your system, where the cost varies. This is a great option, and works very well. If you are changing cost after the PO has been committed, then that really negates the idea of the PO itself.
With the Inventory Wizard - you can change your cost line item by Task 130: Change item costs. You can either go at this by Dept, Cat, Supplier, or a filter. This will present your line items matching your criteria above. You will see your New Cost, New Last Cost and Replacement Cost.
Let's say for example, you would like your new cost to be $0.50 higher than your current cost. To achieve this, you would right click on New Cost> formula> Set New cost to: Cost + .5> rounding optional> verify the example is correct> select to apply to only that one cell, or all rows in the column.
The way I have looked at this in he past is: if a majority of my items will be $0.50 higher in cost, I would apply to the entire column and modify only the few that are not a part of my majority rule, cutting the time down.
I would say the third way of doing this would be a little more manual by way of a report and drilling down. Ron the Reports> Items> Value List> set criteria if applicable> Remove the 'filler' - but not the Item Lookup Code (unnecessary info so it is more easily read)> On the items you wish to change, mouse over the Item Lookup Code, until you get the magnifying glass> double click> brings to the item card for change> OK to save changes.
There are tons of options here - but I would get your costs in order 1st with the Wizard, then move forward with the PO process of changing costs before committing - but remember to set your File> Config> Inventory tab to use the weighted average - this will also play in with transfers.
I hope this is not too convoluted here.