"Set Copy as Master" only seems to work on one photo at a time, even when selecting multiple virtual copies in the Library Grid view. How do I change them without going to the menu one photo at a time?
I can't think of any way to do it, en masse, without a plugin.
You can use Relative Antics (without resorting to advanced settings / customization) to sync virtual copies to masters, and/or vice versa - perhaps that would suffice. If not, then to do a true swap (as 'Set Copy as Master' does), you'd have to go through a temporary set of virtual copies, i.e. create another temp set of virtual copies with same settings as masters, then:
Transfer settings from virtual copies to masters.
Transfer settings from temp copies to virtual copies.
Delete temp set of virtual copies.
This could be automated if you can do a little lua coding.. (or I may do it at some point..).