freakshow10mm is correct.
California's laws govern guns AS ASSEMBLED. An upper can be used as part of a California-legal gun in many ways. There is no "constructive possession" in California law, either. So a non-NFA upper, by itself, can't be illegal under current California law.
It is not legally a firearm.
You can send an upper to California, same as to any other state (i.e. subject to NFA regulations, etc.).
An
assembled receiver has to be put together in compliance with California law, but a stripped lower (through FFL), as well as parts for it, etc. (no FFL needed) are all legal to ship and receive in California.
I built a couple ARs in California, and studied the laws very closely when I did it.
If you have any questions, go here:
http://www.calguns.net
Unfortunately, THR is full of misinformation about California gun laws.