The product detail page should have the option to show when/where the product is used.
For example past invoices the product was added to or current reoccurring invoices to make it easier to track down changes on products and services.
For example: Product Description of Business Standard from Microsoft
Product: in use in (5) recurring invoices
List of the invoice modules:
If you update the price on the product it should prompt you to see if you want to update the price in the modules as well.
Just an idea but it would save a lot of time.