In the Philippines, a person’s name is not merely a label but a matter of public interest, governed strictly by the Civil Code of the Philippines, Republic Act No. 9048 (as amended by R.A. 10172), and prevailing jurisprudence from the Supreme Court. Changing a child's last name is not a matter of parental whim; it requires a specific legal basis and a formal process, depending on the child's legitimacy and the reason for the change.

I. Legal Basis for Surnames

Under Philippine law, the surname a child bears is primarily determined by their status at birth:

Legitimate Children: Those born during a valid marriage. They shall principally use the surname of the father.
Illegitimate Children: Those born outside a valid marriage. Under Republic Act No. 9255, illegitimate children shall use the surname of their mother, unless their father has formally recognized them through the Record of Birth or an Admission of Paternity.

II. Grounds for Changing a Surname

The courts and the Office of the Civil Registrar do not grant name changes for trivial reasons. The common grounds include:

Legitimation: When parents of an illegitimate child subsequently marry (provided there were no legal impediments to marry at the time of conception), the child's status is elevated to "legitimate," and their surname can be changed to the father’s.
Recognition of Paternity: Under R.A. 9255, an illegitimate child who originally used the mother's surname may switch to the father’s surname if the father signs an Affidavit to Use the Surname of the Father (AUSF).
Adoption: Upon the issuance of a Decree of Adoption, the child loses the surname of the biological parents and legally assumes the surname of the adopter.
Avoidance of Confusion or Ridicule: If the surname is tainted with dishonor, is extremely difficult to write or pronounce, or causes undue confusion.
Consistency with Reality: When the child has been known since infancy by a different surname than what is on the birth certificate, and changing it serves the child’s best interest.

III. The Two Main Procedures

The process for changing a surname depends on whether the change is considered a "clerical error" or a "substantial change."

A. Administrative Correction (R.A. 9048/10172)

This is handled by the Local Civil Registrar (LCR) where the birth was recorded. It is faster and less expensive than a court case.

Applicability: For correcting clerical or typographical errors (e.g., a misspelled surname) or changing a first name.
Note: Generally, changing a surname based on status (like moving from mother's to father's name) is handled under specific laws like R.A. 9255, which is also an administrative process at the LCR.

B. Judicial Change of Name (Rule 103 of the Rules of Court)

If the change is "substantial"—meaning it affects the civil status or legitimacy of the child—a petition must be filed in the Regional Trial Court (RTC).

Process:

Filing of Petition: Filed in the province where the child resides.
Publication: The court will order the petition to be published in a newspaper of general circulation once a week for three consecutive weeks.
Hearing: The Solicitor General or the Provincial Prosecutor will represent the State to ensure the change is not being sought for fraudulent purposes.
Decision: If granted, the court issues an order to the Civil Registrar to amend the birth record.

IV. Specific Scenarios

1. Changing an Illegitimate Child’s Name to the Father’s

If the father is willing to recognize the child, the parents can file the AUSF at the Local Civil Registry Office. If the child is under 7, the mother signs; if between 7 and 17, the child signs with the mother's attestation; if 18 or older, the individual signs themselves.

2. After Annulment or Declaration of Nullity

If a marriage is annulled, the children remain "legitimate" in the eyes of the law. Therefore, they continue to use the father’s surname. The mother’s reversion to her maiden name does not automatically change the children's surnames.

3. Step-parent Adoption

A step-father cannot simply "give" his surname to his wife’s child from a previous relationship. The only legal way to change the child’s surname to the step-father’s is through a Petition for Adoption.

V. Essential Requirements

While requirements vary by jurisdiction, the following are typically mandatory:

PSA Birth Certificate of the child.
Affidavits of Two Disinterested Persons (who can attest to the child's identity).
Clearances: NBI, Police, and Prosecutor clearances (to prove the name change isn't to hide a criminal record—usually for older children/adults).
Proof of Paternity (if applicable, such as the Private Handwritten Instrument or Admission of Paternity).

VI. Summary Table: Choosing the Path

Situation	Legal Mechanism	Venue
Misspelled surname	R.A. 9048 (Administrative)	Local Civil Registrar
Illegitimate to Father’s Name	R.A. 9255	Local Civil Registrar
Legitimation by Marriage	Rule on Legitimation	Local Civil Registrar
Change due to Adoption	Domestic Administrative Adoption Act	NACC / Court
Substantial Change/Confusion	Rule 103 (Judicial)	Regional Trial Court

Important Consideration: The "Best Interest of the Child"

In all cases involving minors, Philippine courts prioritize the "Best Interest of the Child" doctrine. Even if a legal ground exists, the court may deny the petition if it determines that changing the name would cause the child psychological distress or confusion regarding their identity.