pctoolbin

Well if that's the case, you can set it to exclude the admin group. So set the following settings as such:
 
Primary Group = Donors
Primary Group Exclude = Select admin groups
Secondary Groups = "Regular User"
 
That way the normal user groups will be upgraded to Donors group with regular users still set as their secondary groups. Admins would be excluded from any upgrades and would needed to be upgraded manually.