Try reviewing the BOM Global Replace feature in Manufacturing / Activities / Bill of Material to see if it's suitable. That sometimes depends on the complexity of your routings, but if don't have a dedicated test database, you could verify the results by running it against a single routing. This will affect only your routings though not work orders, since those represent active transactions. Any WO modifications would have to be performed manually, and if there are UOM or other quantity-based differences, you need to manually adjust the routings. You would generally want to run a BOM rollup afterward to adjust the standard costs of existing inventory, but that's optional. Replacing an item in a routing will automatically adjust the BOM for that routing. The utility doesn't work for every type of routing configuration, but it's worth a look.