In the last month Their poll has moved 8 or 9 points in sample from R to D 
...which simply means that more people are identifying as D now than they did then -- which is no doubt why Obama has moved ahead in the polls. One or two polls might be an exception, but almost ALL of them are showing this trend. Party ID is not an immutable trait. It's a self-identification based on a respondent's political leanings AT THAT MOMENT.
This is how random samples of voters -- many such samples by now -- are answering. What is so difficult to understand about that?
