In the 'Callan' episode "A Village Called G", Section head "Charlie" Hunter said that his secretary Liz could have been abducted by KGB, East Germans, French.
David Callan asked, "CIA?"
There may have been a very good reason that Callan asked that question, beyond reminding the others that even Britain's allies would use espionage against them.
Callan could have been thinking about his brother Robert McCall, a "Company" operative. (It would be David who would have changed his name.)
There may be some who dabble in televisiology like me own self who would rather put forth the theory that David Callan and Robert McCall were the same man. I couldn't do that, if for no other reason than that I've only seen half a dozen episodes so far, and certainly not the final movie, "The Wet Job". I have no idea what happens with the man and I don't know enough about his background. (I know slightly more about McCall, but I'm hardly an expert.)
But having Robert McCall be Callan's brother would eliminate the need to worry too much about the differences in their background history. David mentioned in "Suddenly - At Home" that he didn't attend the "swell" schools like Eton or Harrow, which may have been his choice. This may be invalidatd by some other episode, but I get the impression that the Section recruited him out of the Army, probably because of his marksmanship skills.
On the other hand, Robert McCall probably did go to university and developed a healthy love for the arts and other fine things. He also was probably recruited out of university - if not by the Company outright, then by some other intelligence agency before transferring to the Company.
Robert may have worked in the "Section" like his brother David, which would be reason enough for David to change his name - so that the family connection couldn't be used against either one of them. David might have chosen "Callan" because it was the closest to paying tribute to his murdered father, Captain William McCall. It sounds like a diminutive of "McCall", in a way.
Robert also had a family of his own: a daughter (Yvette Marcel) by a former lover, and a son (Scott McCall) with his wife. (Another daughter, Kathy McCall, died at the age of 18 months.) It would have been bad enough that they might be targeted because of their relationship to him; so that could have been another reason for David to change his name, in order to protect his niece and nephew.
As always, this is speculation on my part and not established fact.
One final O'Bservation: since it's inconceivable to me that anyone else could play either David Callan or Robert McCall, I'd rather think that both of them have passed away off-screen in Toobworld (as did the actor who played them, Edward Woodward) than to ever see the roles recast.....
TV Acres has an excellent timeline for Robert McCall's life and I see nothing in there that could turn into a Zonk for this theory of relateeveety.