@graz: This is weird – as far as I know, when you install a new ROM, you’re writing it over the existing ROM, thus unconditionally wiping the latter.
@Manoj: Would you mind elaborating on why you think there is a version dependency between the existing and new ROM?
@archie: I think answering your question here makes more sense given the discussion: I went stock Pie > LOS Pie > /e/ Oreo.