B737-800
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Time Pressure
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors; Procedure
We were expecting to land runway 35R at DEN.  We had already set up and briefed the ILS 35R CAT III.  While descending through FL180 we were told to expect ILS 35L.  We then set up and briefed the ILS 35L CAT III. We were on the BOSSS2 RNAV arrival which transitions into the approach. As we passed BOSSS we received an FMC alert saying the approach and frequency does not match. The airplane started turning right and descending for ILS 35R instead of staying at 12;000 feet like it should have for 35L. We immediately selected altitude hold and heading select and turned back on course. We told Denver Approach what had happened and asked if we needed to climb back up to 12;000 feet. He said it was no problem and told us to descend and maintain 10;000 feet.While re-briefing the approach we did not catch that the FMS still had the previous approach loaded. A contributing factor was that we are still trying to get familiar with our new checklists and flows. This runway change happened last minute while we were performing our descent checklist and before landing checklist. Denver field elevation is 5;434 feet; so our before landing checklist that begins at 10;000 AFL came quickly after passing FL180.When briefing an approach; always make sure the FMS is loaded with the correct approach.
[Report narrative contained no additional information].
B737-800 flight crew reported a track deviation occurred on the BOSSS2 RNAV arrival into DEN when the FMC was improperly programmed following a late runway change.