Nice Article. I learned a lot. But for this problem there may be a shorter/ simpler solution :

--======1 Insert Records We want to have

Insert into SOURCE(Name, Age, Sex)

Select Name

, Age

, Case Sex When 'M' Then 'N' Else 'G' End

From SOURCE

Group By Name, Age, Sex

Having Count (*) > 1

--======2: Remove the old rows

Delete from SOURCE

Where Sex In ('M','F')

--======3: Update the inserted records accordingly

Update SOURCE

Set Sex = CASE Sex When 'N' Then 'M' Else 'F' End

Both of us depend on the case that Sex column has one of two values. If that is not the case then solution would be appending a character / string that does not exist in the original data (ex '~~~~~') in first step. Delete the ones that does not have appended string in the second step. Remove appended string in the third step.