Hi Ellery
Parts with Z in their name are revision 1 silicon. Without Z they are revision 2. There are a few differences between the two but usually nothing that will cause a big problem. You can test the new parts and if there is something that doesn't work you can check the difference and make the necessary code change.
Revision 1 parts will also have more errata so check these - especially, you might need to disable flash pre-fetching and cache aliasing. To be able to use both rev. 1 and rev. 2 parts in a design with a single firmware you may need to build for the rev. 1 and identify the part to make a couple of code choices (this is not always the case - it depends on which modules are actually used).
It seems that Z parts are cropping up in the supply chain due to the fact that there are shortages of many parts (possibly there are old stocks that were destined to be trashed - you may find they date from 2011 or 2012 since rev. 2 replaced rev. 1 about then).
Mark
uTasker developer and supporter (+5'000 hours experience on +60 Kinetis derivatives in +80 product developments)
Kinetis: http://www.utasker.com/kinetis.html
