John,
Your point that Foucault later "regreted" reducing power to truth seems to
imply that there is a change in direction in Foucault's thinking on these
topics. I don't see a change from the interview you note "On Power" from
the essay "Truth and Power" in which you claimed that Foucault was reducing
truth to power. I think the point has always been that knowledge and truth
cannot exist outside the circulation of power.
If anything, there seems to be more emphasis on power later in his career.
Genealogy increasingly became more prominent in Foucault's work, displacing
to some extent his earlier notion of archaeology. I would compare the
analysis in the Order of Things with Discipline and Punishment and the
History and Sexuality (Of course, Foucault never completely rejected
archaeology).
Your suggestion of a "turn" in Foucault made ambiguous the word "reduced."
For example, idealists could look at Foucault's power/knowledge relationship
as a reduction of truth to power, even if truth and power are not one in the
same, as you note. To reduce can mean to attenuate, to lessen; it does not
mean necessarily to be subsumed.
So my question still is:
If there is a genesis of Foucault's thinking on power and truth (knowledge),
when did it occur and why? What is Foucault's "regret?"