employment agreements

[http://dummy.us.eu.org/robert]

I will be getting a full-time job at a large company in Ca.  I'm wondering: should I worry about working on open source software outside of work?  My understanding is that Ca offers some protections as long as the employment contract doesn't override 2870(a) of the Ca Labor Code.  It is also my understanding that it is difficult, if not impossible, to change an employment agreement when signing on with a large company.

Thanks.