A lively conversation has sprung up in response to both Claudia's and Megan's recent posts on changes in the way Ashtanga (in particular, the primary series) is practiced. I thought I'll jump in here as well, and give my two cents' on this matter.
I have noticed these changes as well (particularly, the change from nine rolls to five in Garbha Pindasana), but I have never found them to be shocking or dramatic. But then again, that's probably because I am a relatively new Ashtanga practitioner (I only became a "full time Ashtangi" sometime in the middle of 2009), and my teacher (now you know who he is, he who shall not be named :-)) only got authorized a couple of years ago. By that time, the five-rolls-instead-of-nine rule in Garbha was already well-established. And although I had dabbled in Ashtanga before 2009, and vaguely remembered that it used to be nine rolls and not five (doesn't this sound delicious? Rolls? As in bread? :-)), since I was only a dabbler, the change did not really bother me much.
Actually, come to think of it, the change that bothered me more was the change in drishti in the primary series forward bends from the navel to the toes. I clearly remember being at Tim Miller's workshop in Miami in May 2008, and being told by him that the drishti in Paschimottasana and all the primary series forward bends is the navel, so that in all those postures, one ends up doing a semi-Jalandhara Bandha with the chin and the chest. This agreed with what I had been doing in my own Iyengar-inspired practice up to that point. Mr Iyengar doesn't talk about drishti in his Light on Yoga, but I'm quite sure that in Paschimottasana, he is photographed with his chin down towards his chest.
And then I met my teacher in 2009, and one of the first things he corrected me on was the drishti in all those forward bends, telling me that I should be looking at the toes. I told him about what Tim said, and his response was, "I always go with what Sharath says." End of conversation.
It took me a while to get used to this looking-at-the-toes-in-the-forward-bends thing, but I eventually got used to it. And once I got used to it, I saw that it made a lot of sense from an energetic point of view. I feel that looking forward (as opposed to looking down) has the effect of lifting one's energy, making it easier to transition to the next pose, whereas looking down has the effect of drawing the energy down/inward, so that one has to exert more of a psychic effort to draw the energy back up to transition to the next posture. I don't know if any of this makes sense from any objective point of view, it's just how I feel subjectively.
As to Megan's question, "Why and how does the series get changed?", well, I honestly don't have any answer to this question. My own take on all of this is quite un-Ashtangic, "If something works for you, do it. If it doesn't, try to make it work. If it still doesn't work, well... then do whatever it is that works for you." I mean, what's the worst that can happen? :-) It's not as if some Ashtanga Police are going to barge into your practice room/shala and strip you of your Ashtangic powers if you don't do things the "right way"...