The analysis scripts you used with yt 2.6.1 are going to need to change a little bit to work in yt 3.0.  The biggest difference is how fields are named.  For example, "Density" has become ("gas", "density").  There is a whole page in the documentation devoted to the differences between yt-2.x and yt-3.0.

I recommend checking that out.  If you find something that is not covered, please let us know!

Britton