Since you're restraining access to DT2 this way, yeah it turns into an oligarchy by definition since it will be a smaller number of members with deciding power, and don't argue with me argue with the dictionary 
You like to be categorical.
Where does the dictionary say that 500 is not an oligarchy and 250 is?
I don't see any of that:
https://www.merriam-webster.com/dictionary/oligarchy
Those who are not now in DT could say the same, that the 500 in DT2 plus those in DT1 are an oligarchy.