It doesn't matter if the councillors are old or new - they will have all been elected (an not co-opted) after the 2nd May. As long as you have filled the 2/3rd of your seats then that part of the criteria is met. The clerk needs to be qualified and a resolution passed. That resolution will stand for the 4 years until the next election - it doesn't matter if councillors leave and you end up co-opting more.
However, you have to pass the resolution, GPC will expire otherwise.