Can the QIAcuity One 2plex be upgrade to the QIAcuity One 5plex?

No. The QIAcuity platform introduces four variations: QIAcuity One 2plex, QIAcuity One 5plex, QIAcuity Four (5plex), and QIAcuity Eight (5plex). All of them have fix channel combination.