Technically, yes. Existing OBDeleven users can use your referral link.
Just remember: The referral rewards apply only when they purchase our OBDeleven 3, OBDeleven 3 PRO, or OBDeleven 3 ULTIMATE Pack. So, if they consider gifting a device or getting an extra one, it's a perfect fit.
However, the referral program doesn't extend to digital products like Plans and Credits.