I asked the same this is what they told me.
Name has been withheld to protect the innocent
Dear dogrobber13,
Thank you for using the Ruger On-Line Customer Support Request Form.
This e-mail is in response to your question or comment of 06/27/2012
Request No: 144889
Comment / question:
Is it possible to upgrade a 556c to 556cla configuration?
I hace a 556c w 4 rail handguard and want the Smooth 556cla handguard.
Thanks for your time,
dogrobber13
Response:
This would require a new upper. At this time, we do not offer the CLA upper separately. Please keep checking our web sight for updates, new models, and availability. Thank you for your interest in Ruger firearms.
If you need further information, please visit our website at www.ruger.com
So i am SOL