Downgraded from in general means that the CPU wasn't able to maintain full
stablity at that given rating with default voltage, so it was remarked to a
lower PR. For example 1700+ downgraded from 2400+ means that this CPU can't
work at 2400+ with voltage 1.6V, but it can probably hit even higher
frequencies than 2400+ with voltage increase
But there are rumors that AMD restamped the markings of some high-speed CPUs
to fulfill the HUGE demand for lower speed ones (mostly 1700+ and 2100+), if
this is your case, then the CPU should be able to work stable even at 2400+
with the default voltage.
*This information is not official and therefore I can't guarantee that it is
100% correct or true, it is not based on my personal observation I've found
info on this at one malaysian (I think) forum, but my personal experience
shows that there are processors that fit in the both categories described
above.
About the Bartons, well I believe that these numbers (eight and nineth
symbol) are not used for that purpose, or not used at all. Because the
Barton core is too new and pricey and there in not much demand for it
so there in no need for downgrading procesors or maybe AMD had
improved it production process and there are no CPUs with Barton that
are not stable at their default voltages. I'm not sure about that.
And what if the two digits don't represent the PR rating that the CPU
is downgraded from, insted they may show what is the maximum frequncy
in GHz that the procesor can work stable at when overclocked. Then
this would mean that Athlon XP CPUs with Barton core can work stable
at frequencies up to 2.6 GHz. Anyway thats just an idea...