Can a user of a patented product make changes to the product for their own use (not reselling product) in their business ? From drilling holes to lighten weight or making their own parts for the product to make it work better. Can the patent holder legally prevent this in USA?
In general, once a product is legitimately sold in the U.S., the patent holder has exhausted any and all patent rights. The purchaser can do anything they like with the object. Exceptions would include situation where ownership doesn't transfer like packaged software. It is licensed with conditions. In oral arguments in a Supreme Court case the justices seemed to agree that if a seller wants to put conditions on a sale of an object they should lease the object, retaining ownership of the item.
A separate question involves the rights of third parties. If Smith buys a cup from Jones and drills three holes in the bottom that would not be of any concern to Jones. However Mr. X may have a patent to cups with three holes. Theoretically, Smith can be infringing Mr. X's apparatus patent. Also, Mr. Y might have a method patent on using a cup as a planter for orchids. So, with no modification at all, Smith might put the cup to a use that is covered by a patent.