  Since August 2000, the stock market in the USA as well as most other western
markets have depreciated almost in synchrony according to complex patterns of
drops and local rebounds. In \cite{SZ02QF}, we have proposed to describe this
phenomenon using the concept of a log-periodic power law (LPPL) antibubble,
characterizing behavioral herding between investors leading to a competition
between positive and negative feedbacks in the pricing process. A monthly
prediction for the future evolution of the US S&P 500 index has been issued,
monitored and updated in \cite{urlprediction}, which is still running. Here, we
test the possible existence of a regime switching in the US S&P 500 antibubble.
First, we find some evidence that the antibubble has exhibited a transition in
log-periodicity described by a so-called second-order log-periodicity. Second,
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