This is what LA sent me in response to this Question about the 24% of voters who are registered No Party Preference.

Question: If you registered with no party preference and If you vote by mail, AND you did not return the postcard to select a party ballot - can you bring the ballot you got and exchange it for a DEM party on and vote for Bernie Sanders at your voting precinct?. What else would a voter need to do?

LA County RESPONSE
Thank you for contacting us with your inquiry. Yes, nonpartisan voters may surrender their mailed nonpartisan ballots at the polls, in exchange for a crossover precinct ballot. Please note, these voters may still submit a request to have the crossover ballot mailed to them instead, if they wish. The last day to submit a request is May 31, 2016. The form is available on the crossover voting page of our website at: https://www.lavote.net/home/voting-elections/voting-options/crossover-vo... (see item #4 under "What To Do Next").

Please feel free to contact us again if we can be of further assistance.

Kind Regards,
Voter Info Representative
www.lavote.net

great Reddit Link! on CA Vote Issues
good C99 diary on CA vote issues
link to Federal Elections lawsuit filed against CA SOS for CA Elections