The only thing that's changed since NS0-154 to NS0-155 is the new features that 8.1 brought,
If you know those, you should be good.
Here are some highlights,
- 64bit aggrs
- FilerView is gone
- Flash pools
- Flash Cache (not PAM cards anymore)
- Vol move
- Aggr / Vol maximums that changed with 8.x
- Data compression
- New controllers that depend on 8.x (2240)
- ESH / ESH2 modules for DS14 shelf are no longer supported
...I can't really think of anything else off the top of my head, but if you go to support.netapp.com > documentaiton > Data ONTAP > 8.1.2 7-Mode
There is a section that will tell you all of the new stuff that has been released with 8.x, a lot of stuff changed after 8.1.x, and I doubt it made it into the test, but if you understand all the changes in 8.1.2, you're truly ready for the test.
Then get ready to embrace C-Mode, which has drastically different material, but is where everything is going. (I think getting yourself prepped and able to pass the 7-mode test is a good idea though)