Date: 202106
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Other unknown
Flight phase: Initial Approach
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Fatigue; Distraction; Communication Breakdown; Situational Awareness
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Air Traffic Control; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Received a runway assignment change from Runway XX to Runway XYR after initial Runway XX briefing.  Re-briefed the Runway XYR ILS and began approach.  Between ZZZZZ and ZZZZZ1 intersections PF began descending to 1;500 feet.  Altitude deviation was noted and a correction was made around 1;600 feet; shortly thereafter the tower issued a low altitude warning.  Altitude was corrected back to 2;000 feet; approach and landing was accomplished without further incident.  During debrief; crew agreed that expectation bias was a major contributing factor to the deviation.  The PF was still thinking about the altitudes for the Runway XX ILS.  It was the last leg of a three-day pairing so the PM did not properly back up the PF due to Pilot Flying's strong performance during the entire pairing.  PM should not allow previous performance or experience to impact critical backup role.


Second narrative: Initially set up and briefed for the ILS XX. New ATIS was obtained with runway change to XYR just prior to runway transition point on the ZZZZZ Arrival.  Re briefed and set up for ILS XYR. Approach Control gave us a  vector to intercept the Localizer XYR and then to descend to 2;000 feet from 4;000 feet outside of ZZZZZ [intersection] and then cleared us for the approach. I armed the localizer and set 2;000 feet; localizer captured and upon reaching 2;000 feet at ZZZZZ; I reverted to my previous briefing of the ILS XX and set 1;500 feet and proceeded to descend to 1;500 feet. Several hundred feet into the descent; Approach Control told us that we were low. Upon realizing what I had done I initiated a climb back to 2;000 feet from now 1;600 feet prior to ZZZZZ1. Reaching 2;000 feet prior to ZZZZZ1 and being stable again; we now continued to configure and resume the approach to land with no further incident.I should have verified my actions as I was accomplishing them and verified them with the PM. I was still in the mind set of what I had originally briefed and should have transitioned to the new approach even though we were rushed with the late runway change.


Synopsis: Air carrier flight crew reported receiving a low altitude alert from ATC.