bbmouse,
Hmm, there are several things in consideration for BPA. Firstly, its actual toxicity. It has been tested in lab mice at various concentrations and in some studies proved to be harmless to reproduction even at high levels (but do other damages). There are some studies that showed it may exacerbate cancers. It is found to have higher potency in the test-tubes then in-vivo so maybe the body has some way of metabolizing (neutralizing) it. Again, all these studies done in mice. Remember their size in comparison to humans. I dun think any of these studies did a relative size - dose thing.
Secondly, the other thing to consider is the release of BPA. How much of it really leech from the bottles into the water? I haven't seen much studies with regards to this but then I haven't got the time to even look through all the titles yet.
Hmm, my conclusion in my brief 10 - 15 mins research is that, if you have the money, go ahead to change for a peace of mind, if you dun have the money, dun need to feel bad at all about not changing cuz it may well that BPA is not toxic to humans at whichever level it is leeching into the water. Besides, we have all been exposed to it already, one way or another.