These substitutions don't appear to be weather/irregular ops related.

Is there a big event going on in SUX this week?

The DC-9-50 upgrade from the CJR on the last flight of the night for MSP-SUX is listed in the online schedule for Monday 1/12 & Friday 1/15 this week, with the return on the first flight SUX-MSP Tues. & Saturday morning.

The CR9 is not, but since XJ operates most of the Saab & CRJ flights into SUX it appears as they performed and aircraft swap for loads.

It was not uncommon for NW/XJ to swap a Saab for a CR9 on DTW-FNT, particularly if they had cancelled flights on the previous day.