Yes, it's common practice to amend the names of one or both birthparents on an amended birth certificate. The names of the birthparents appear on the original birth certificate, which would normally be sealed and kept on file at the Georgia Dept. of Human Resources, Vital Records Service, 2600 Skyland Drive NE, Atlanta GA 30319.
That office also has marriage records, and I assume your birthfather's name, age, names of his parents, etc. are on your mother's first marriage license record.
For the education of others involved in Georgia adoptions, let me mention that I received an e-mail from the Evan B. Donaldson Adoption Institute on 2/27/04 that said that birthparents are permitted to access the Georgia Adoption Reunion Registry as of 7/1/03---based on a new law.
That registry is said to be at the Dept. of Human Resources, Division of Family & Children Services, 878 Peachtree St NE, Atlanta GA 30309.
Reg Niles
RegNMINiles@cs.com