As I’ve changed my solo players to section players in Setup Mode, my player designations have changed from Arabic numerals to Roman numerals. How do I change this to all Arabic numerals?
Provided you have Dorico Pro, you can do this as described here.
The left list of Engraving options.
If you’re ever not sure about this sort of thing, you should find it described/labelled on the page for the corresponding area or dialog, e.g. here for Engraving Options.