You can execute affidavit that "Udal Singh" and "Udal Singh Kashyap" refer to one and the exact same individual.

Mention his official death certificate, his Pension Payment Order (PPO) if applicable, and your own identity documents showing the surname

2) public notice must be issued in two prominent local newspapers (one English national daily and one regional vernacular newspaper).

3) The Central Government requires a Gazette of India Notification to legally acknowledge changes or additions in service records.

4) submit application addressed to the Head of Office (HoO) / Appointing Authority of the Ministry or Department where your father was employed

One can change or surname at any point if time and no law prohibits that. The problem arises when the change of name may not appear in the public documents like Adhaar, PAN , Passport etc which you rely upon at the hour of need.

Therefore, after change of name or surname you have to bring the changes in those documents and publish in newspaper.  

- One can declare his name /change his name after affirming it in an Affidavit and after publishing in news papers 

- However , if you want to change his name or to add Kashyap in his name , then it cannot be possible after his demise . 

- If you have any another document with the said surname , then on that ground it may be possible.