Okay so to be most correct when converting an epub2 to an epub3, ePub3-itizer should then recognize and convert calibre:series and calibre:series_index inside epub2 style meta entries to:
Code:
<meta id="num" property="belongs-to-collection">Series Name Here</meta>
<meta refines="#num" property="collection-type">series</meta>
<meta refines="#num" property="group-position">1</meta>
I will add support for that.
ps: I have now created fixes for this which I will push tomorrow to support recognition of calibre:series and calibre:series_index in both the ePub3-itizer plugin and inside Sigil itself inside Sigil's Metadata Editor GUI when used on an epub2.
pps: Sigil master now has support for calibre:series and calibre:series_index in the epub2 Metadata Editor.