Value Category is to group the cost based on Cost Element or Cost element group assigned to that Value category.
For example, you have "Value Category A" with some Cost element 50000. In PM Order, if you book any cost on 50000, it will be displayed as Value Category A. If you want to discard "Value Category A" & you want to show the the cost booked in 50000 with "Value Category B", then remove the assignment of 50000 from "Value Category A" & do the assignment to "Value Category B".
Then run program RIPMCO00 in SE38 with appropriate details, so that cost will be shown in "Value Category B" which had been shown in "Value Category A" previously.