The last time this happened to me, after DL MX delays on a domestic connecting flight would have caused me to miss a DL TPAC, the SC agent seemed willing to rebook me into business class on some flights but not others, both DL and parners. I suspect the rule starts with whether the DL flight has available D1 seats and then is willing to rebook into a partner's (deep) discounted business class, but not partner last seat availability or something like a J or C inventory class. I'm DM which might have helped so YMMV. Also, my flights were M class upgraded at the time of ticketing with an old SWU.