Dave:
Try this (it worked in AC17):
1. Give the current Custom Layer Combo a name.
2. Delete the old named layer combo
3. Change the name of the renamed custom combo to the old combo name
I found that this would replace the Layer Combo for the View Settings, but it did assign a new Index Number (in the Attribute Manager). You should test on a copy of the file to see if it works in your case.
David