Yes. First, it is not clear from your question whether a default has been taken against the defendant or defendant has simply failed to answer within 30 days. If latter, the mere fact that the defendant has failed to file a timely response does not indicate that a default has been entered against him. (You need to take additional steps in order to have a default enterered.) Second, even if a default has been taken, an amended complaint making substantive changes in the cause of action against a defaulting defendant supersedes the original complaint. In effect, it "opens the default" because the plaintiff is required to serve the amended complaint on the defendant; and the defendant is then entitled to answer or otherwise respond to the amended complaint.
Disclaimer: This response is for informational purpose only and shall not be construed as a legal advice. This response does not create an attorney-client relationship.
Yes, the amended complaint must be served on the defendants. And as Attorney Yee indicates, defendants would now have the opportunity to answer the amended complaint, notwithstanding the default on the original complaint.
The amended complaint cures the default. In other words, if you amend the complaint as to the defaulted party he is no longer in default. You have to serve it all over again.
I would add that the Defendant in this scenario should be personally served the Amended Complaint if you do not have proof of personal service on the original Complaint. Ordinarily when a Defendant has been personally served the original Complaint, the Amended Complaint can be served by mail. But in this scenario, where there was no response whatsoever, perhaps there was never successful service of process of the original Complaint. Make sure that service of process on the original Complaint was successfully accomplished or consider seeking local counsel to discuss whether it should be re-served personally.