Hi Nancy, I doubt that there was ever an actual name change. I think the spelling just evolved depending on who was spelling it at the time. I find several variations in census records and other documents.
A Widow Fertick appears in the 1790 Federal census in district #96 in SC. The 1800 census lists a John Firdigg in Fairfield Co., SC. (I assume this is our John George Furtick since he is listed with several young children in the household.) The 1820 census lists George Ferdict in Lexington County. By 1820 the spelling on that census is George Furtick. In the 1830 census George's widow is listed as Mary Fertick. After the 1830 census, though, the name seems to be spelled mostly Furtick in SC census and land records.
The only exceptions that I have found is David Furtic, 1850 Lexington Co. and William Festick, 1840 Orangeburg Co. I believe these are both George and Mary's sons.
In 1870 the family name is spelled Furlick in the census index. I have not seen the actual records.
Do you have a list of George and Mary's children as shown on their estate records?