I figured out why my estimate was reverting to an old invoice template-- it hadn't been made inactive.  Once I did that, the estimates are now going to the one active invoice template which is the correct, updated one.  For others with this issue, check to see if you have older format templates that have not been made inactive (lists, templates, and see if you have more than one version of templates still active.  Deactivate the one you want to "kill off."  Hope it works for others.  Surprised this simple fix has not occurred to QB to suggest.