Well, at least you stopped me messing around with 'add traces', trying to copy values into the trace properties pages, and other irrelevant things, so thanks for that.
After another couple of hours, I finally twigged it - it is easy, but the sequence is not obvious. For other simple folk:
1. Run simulation 1, set the trace and graph properties how you want
2. Change a component value, run simulation 2
3. Select the first graph, then CLICK IN THE GRAPH AREA
4. Now 'copy graph properties' is available - it isn't available if you don't click in the graph area first. Click it.
5. Select the second graph, then CLICK IN THE GRAPH AREA
6. Now 'paste graph properties' is available - it isn't available if you don't click in the graph area first. Click it.
Failing to click in the graph area in either step 3 or 5, and then just selecting copy or paste, copies or pastes the entire graph, including the actual values, which we don't want (in this case).
This stuff is kind of obliquely implied in the help - or at least, once you know how to do it, the help makes sense. I don't know why they don't just tell you how to do it though.
