The default functionality of Megaventory restricts the update of the Bill of Materials of a product of type Manufacture from Work Order with at least one product being in a Work Order that is neither Closed nor Cancelled.
If all Work Orders associated with a given BoM are either Cancelled or Closed you can update the BoM. You should however understand the consequences when doing this: If you edit the BoM of a manufactured good with existing stock and then go on to manufacture again based on a new BoM, the result would be the same Finished Good originating from two different BoMs. In such a case you would end up with two different products (they are different by definition due to their different components) in your stock which are described in the Megaventory system by a single Finished Good product code.
If at a later day, you wanted to retrieve that Finished Good (to sell it, to manufacture something else from it, etc) you would not be able to differentiate in the system which of the two physical products (ie which of the two BoMs have created the physical product in question) is available. This warning is in place to prevent this confusion - Megaventory will not protect you from doing this so you have to keep track of changing BoMs.
A suggested alternative method to better keep track of your products is to create another Finished Good product code. You can create version B with a similar SKU (say FG001-B) and copy the BoM of the old FG SKU adjusting any raw materials in the new BoM.
When you create the FG001-B, you will be prompted to add the BoM. In the pop-up window click the Copy BoM option to choose the old FG and have the BoM copied in one click.
In case you want to change the default functionality of Megaventory and be able to update the BOM while Work Orders of this BoM are open, you can select this option from the Account Setup screen.
Work Orders of this BoM that are already open will not be affected. However, new Work Orders that will be created will use the updated BoM.