There is evidence in the records that many of these Cowans left the area by 1766.
John Cowan dissappears from the record in 1750-1751 when he sells 202 acres of land to Samuel Lusk:
John Cowan, Yeoman, To Saml. Lusk, Farmer, 202 Acres; Thomas Shield's Corner In Col. Patton's Line; Andrew Alexander's Line; Corner Robert Christian, Jennet, Wife Of John. Teste: Edward Erwin, Hugh Campbell, George Anderson.
Presumably he left the area, but to where we don't know. One possibility is that he moved further south, settling somewhere along the Appalachian front, perhaps as far south as the head of the Holston River. If so, it was an ill-timed move; Indian Hostilities during the The French and Indian War (1753-1763) drove most early settlers out of this area, moving even further south into the Carolinas, or even returning north into Pennsylvania.
James and Andrew appear in the records of Augusta County over a considerable period. Both, however, leave the area at about the same time. In the case of Andrew we at least what his plans were, as a letter of his survives in Augusta County court records, summarized in Chalkley's Chronicles as:
Cowan vs. Cowan.--Letter of Andrew Cowan to David Cowan, living in Albemarle, dated January 13, 1766. Loving Brother: This comes to let you know that we are all in good health at present, blessed be God for his mercies. Hoping that these few lines will find you in the same, I desire that you will go to James Cowan's and ask him for a note of mine that lies in my little trunk--a note of 2 pounds and five shillings, which William Teas(?) is due me against March 25, and get the money. for I am going to Carolina. I have nothing material to write you, but desire to be remembered to you all.
The James mentioned in the letter does not appear to be the James who lived in Augusta County, As Andrew would probably have had no need to ask his brother in Albermarle to do him this favor unless the James referred to was also in Albemarle County. This also suggests that we are dealing with two sets of Cowan's in Augusta; if James of Albemarle is a brother of Andrew and David, than James of Augusta can not also be a brother. Their mutual proximity to each other, both in time and space, suggests that they are at least cousins of one degree or another.
In anycase, we know from this letter that Andrew of Augusta plans on going somewhere in the Carolinas. Other court records show that he is gone from Augusta by 1766
James Bell, provisions. Wm. Bell, provisions. Wm. McCutcheon, provisions and horse impressed. John Rosemond, provisions. Andrew Cowan, enlisting men to garrison Fort Lewis. Walter Trimble, provisions. Thos. Alexander, provisions. John Francis, provisions. James Kirk, provisions. Rob. Armstrong, provisions. Wm. Christian, self et als., ranging. Loftus Pullen, provisions. Rob. Christian, provisions. Danl. O'Freild, provisions. Thos. Poage, provisions. Charles Kilpatrick, provisions. George Moffett, for Wm. Mann et als. Benj. Estill, horse impressed. Andrew Hamilton, provisions. Wm. McClenachan, provisions. Wm. McKarney, self et als., ranging.