I think the switch should be random. As in, one or more numbers are assigned to each race. Then, when a member signs up for the RPG, one of the moderators does a roll on random.org and the resulting number shows which race each member will switch to. I think more numbers for a race would increase the randomness factor, but make sure the number of numbers is equal for each race and that they are equally spaced, to avoid creating higher chances for one race than another.