The jump from riddle tasks to inheritance and first person, together with the new refrain, must have been, I feel, a conscious act probably by one person, as these are all big jumps. Changing refrains are common in ballads even where they use proper words, but the whole meaning and purpose of the song changes in 'Sing Ivy' types. What can be seen is a lengthening of the progression of tasks in Elfin Knight types before this takes place. I think it probably happened some time in the middle of the 19thc.