FYI— I just did the calculations:
SFO-ATL-DTW-MSN is 3226 miles.
SFO-MSP-MSN (the most direct NW/DL route they could put you on) is still 3140 if you get rebooked in Y. So I don't know how close you are, but you may not need original credit.
Or, if you need those last 86 miles, you could check availability and give the GA flight numbers of a routing through DTW or CVG. I can't imagine them saying no.