You cannot change 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 change the BoM. You should however be very careful when doing this.
The reasoning behind this warning is the following. 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.