I also wanted to rename the currency a few days ago with a script, so here you go:
# Put this script below Materials, but above Main.
class Game_Interpreter
def change_currency_name(name)
$data_system.currency_unit = name
end
end
Example for the event:
change_currency_name("Gold")
I didn´t really test this much because I got rid of that idea, so I hope this works for you.