After a valid final sitting, a person mistakenly stands for an extra rak'ah and then performs its Sajdah. What happens to the original Fard?

Sujud al-Sahw: Cause, Method, Timing, and Special Cases

• The original Fard remains valid; the extra portion is completed as a pair of Nafl rak'ahs and Sahw is performed for delayed Salaam.
• The original Fard is automatically erased.
• The prayer becomes Janazah prayer.
• The person must omit all further prayer.