That substitution was actually a result of printing text, and limiting the number of characters used. In the 13th century Scotland this was all going to be written by hand, so y was still an “i” or a “u” and thorn would have been its own character; uppercase þ, lowercase ð.
For an earlier example of y as a u, and thorns all over the place, try some of this intermediate Saxon circa 9th century
Feala ic on þam beorge gebiden hæbbe
wraðra wyrda. Geseah ic weruda god
þearle þenian. þystro hæfdon
bewrigen mid wolcnum wealdendes hræw,
scirne sciman, sceadu forðeode,
wann under wolcnum. Weop eal gesceaft,
cwiðdon cyninges fyll. Crist wæs on rood.
Amusingly, G’s at the start of a word are used like Y’s are used today, Y’s are U’s, and small furry creatures from Alpha Centauri were REAL small furry creatures from Alpha Centauri.
For an earlier example of y as a u, and thorns all over the place, try some of this intermediate Saxon circa 9th century
http://www.sacred-texts.com/neu/ascp/a02_05.htm
Feala ic on þam beorge gebiden hæbbe wraðra wyrda. Geseah ic weruda god þearle þenian. þystro hæfdon bewrigen mid wolcnum wealdendes hræw, scirne sciman, sceadu forðeode, wann under wolcnum. Weop eal gesceaft, cwiðdon cyninges fyll. Crist wæs on rood.
Amusingly, G’s at the start of a word are used like Y’s are used today, Y’s are U’s, and small furry creatures from Alpha Centauri were REAL small furry creatures from Alpha Centauri.