This turns out to be easy to do in D2L, but I was deceived by part of the D2L user interface.
In D2L when you go to the External Learning Tools manager (from Course Admin) all of the LTI links are there. They are clickable, and also there is a dropdown menu to the side. The dropdown menu lets you see the details of the request or delete the link. I assumed actually clicking the LTI link would execute it, but that actually takes you to an edit page where you can edit the launch URL.
So while our courses typically have two dozen of these links, it is not so bad to visit all of them here in the ELT manager and make the small requisite change.