Thanks LN for correcting me here and apologies to others if I have mislead. It was late at night but I don´t know what I was thinking likening it the land – lönd  (a – ö) vowel change in neut nouns in nom and acc plural which of course is only applicable to a. I guess I was trying to explain why the noun was listed as troll in Zoega but appeared as tröll in the text.