I have done a lot of reading here and on other sites and I wouldn't worry. The time to harvest provided by seed suppliers is often much shorter than most actual grows. Their time-frame is generally only achieved by 'rushing' the plant by switching to bloom nutrients as soon as the plant shows it's gender.
As for your concern about THC degradation, you should be fine with white pistils and clear trichs. Most plants' buds will have trichomes that go from clear to milky to amber as they mature but apparently some do not. You could do more research on your strain, if you are still concerned.
My guess is that your girls are so well cared for, that they are taking some extra time to reward you with an extra bountiful harvest. Are you in a hurry to kick them out and make room for your new girls ?