I think it would be best to clarify with your customers what the intended use of the product is. I'm thinking of the parts that we occasionally outsource to a machine shop.
1). A one-time mounting bracket / tool holder for a piece of production equipment or a 5S / process flow improvement
-This likely wouldn't be a problem if there was some slight tweak, as long as all the materials stayed the same.
2). A validated/qualified fixture for processing product through some piece of equipment.
-As long as the final output matches the drawing, it likely wouldn't be a problem. However, I would definitely notify the customer if it's being manufactured in a different method than originally agreed to so that they could assess the risk to their process and product.
3). A piece of an actual medical device.
-This is where it gets really hairy, as yodon alluded to.