After repeatedly running headlong into this issue, I think I (maybe) finally understand. It appears that when creating a new master page based upon another page (First or Default), the things one is able to change are frame positions (e.g., slightly lowered for high treble notes), etc. What I would like to do is create a new master page that looks exactly like another, then alter some characteristic of that new master page, without affecting the one it is based upon.
Here is the exact situation I’m trying to address. I’m a church musician; many hymn concertato settings will create parts which span 2 pages for the instrumentalists. The Default Master Page puts page numbers left-aligned and right-aligned as per booklet printing (first page on the right, second page on the left of the inside spread). All good.
I want to create a new Master Page that looks exactly like the Default one, with the only difference being that the page numbers are swapped left-to-right, and justified accordingly. This is for printing a 2-up spread with the first page on the left, the second on the right. All good.
I can change the frame positioning of the page numbers with little [well, relatively!] difficulty. But… if I change the justification of the page numbers in the new Master Page, it also changes the justification of the “based upon” Master Page. NOT what I what to do - that will affect all the parts which DO need the original Master Page.
So… my question is: how do I create a new Master Page which is a copy of an existing Master Page, to which I can make edits which don’t affect the original page it is based upon? I just don’t want to start from scratch - there should be a way to copy a page layout, then make adjustments to it which don’t change the original. Or am I obtusely missing something?