You have 1.5 mill posts with over 52,000 members and when I checked it out there were 181 members and 84 guests online.
Why change anything when what you have is very popular?
Confusious said, "Tread on water when no sharks are around; swim to safety when in danger."
The answer is simple...
increase registration;
increase productivity (posts);
increase amount of time members stay online;
increase clickthrough;
increase sponsors;
more competition, etc.
Change is good if you do it right.
Change is good if you do it gradually.
Abrupt changes will/can cause havoc.