Hi Emoore
I have been reluctant to post because I am not in the fitness industry, but since there have been a lot of views and few posts, I will give it a shot. I am involved in tertiary education, so to promote my industry, I would say that education is an end in itself.
I say that not to be sarcastic but because even if it does not directly impact your pay scale (I assume this is part of the "is it worthwhile going back for a major" question), it will affect the quality of your professional career and I think directly impact your enjoyment of your profession. However, you do not necessarily need to do it formally because there is no reason that you cannot educate yourself.
I am not too familiar with the US education system and am surprised that you can go back and "add a major" to a degree that you have finished. But rather than "adding a major", I would suggest that you do a completely new program (for example a post graduate degree or certificate - -it is usually the equivalent of 3 master levels courses). Or actually bite the bullet and just do a masters degree.
In any case, since you have apparently found what you want to do, any investment in education will be very worthwhile. Keep us updated on what you decide to do.
Cheers
Peter