An attempt has been made to account for the substitution of names by taking "thorn" to be a kenning (metaphor) for "giant".
The name of 𐌸, the Gothic letter corresponding to Þ is an exception; it is recorded as þiuþ "(the) good" in the Codex Vindobonensis 795, and as such unrelated to either þurs or þorn.
Old Icelandic ᚦ Þurs er kvenna kvöl ok kletta búi ok varðrúnar verr.
Anglo-Saxon ᚦ Ðorn bẏþ ðearle scearp; ðegna gehƿẏlcum anfeng ẏs ẏfẏl, ungemetum reþe manna gehƿẏlcun, ðe him mid resteð.
The thorn is exceedingly sharp, an evil thing for any thegn to touch, uncommonly severe on all who sit among them.