It would be slightly more accurate to say that you receive a new copyright in any new material in the work, not that your copyright is extended on the old mater

It would be slightly more accurate to say that you receive a new copyright in any new material in the work, not that your copyright is extended on the old material. Under U.S. law at least, a significant alteration of a work results in a derivative work under law, and you are correct to say that it is entirely reasonable to place an updated copyright date on that work.