Date: 201405
Local time of day: 0601-1200
Location: ZDV.ARTCC, CO
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZDV
ATC / advisory: Center ZDV
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Human factors: Confusion; Workload; Time Pressure; Situational Awareness; Distraction
Second reporter function: Captain; Pilot Not Flying
Anomaly: ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action; Flight Crew Returned To Clearance
Contributing factors: Chart Or Publication; Procedure; Weather
Primary problem: Ambiguous

Narrative: I was the pilot flying on the JAGGR Arrival into DEN. About 40 miles east of OATHE; the fix starting the JAGGR Arrival; DEN Center cleared us to DEN on the PURRL Arrival. The airport was switching landing directions; going from a south flow to a north flow. About five minutes later; we were cleared back onto the JAGGR Arrival. The JAGGR and PURRL Arrivals share the same step-down fixes and altitudes until after the LYNKN fix. After LYNKN; the PURRL Arrival has lower altitudes given the north landing flow. We were then cleared to FL260. No Descend Via clearance. Somewhere outside DANDD on the JAGGR Arrival; we were cleared again to DEN via direct DANDD and the PURRL Arrival. We were given a Descend Via clearance on the PURRL. After getting the arrival reprogrammed and 9;000 FT set in the mode control panel our VNAV showed us 5;000 FT high on the arrival; a late descent clearance. I reset the altitude to 17;000 FT (the lowest altitude over DANDD) and used Vertical Speed to get us back on path. I then selected VNAV and reset the MCP to the altitude clearance limit of 9;000 FT. Somewhere in all this; we were switched to DEN Approach and told to slow to 230 KTS. While trying to slow to 230; I received a VNAV disconnect message. I reset the altitude in the MCP and selected Vertical Speed and the speedbrakes to attempt to comply with ATC's new speed request and altitude requirements on the PURRL. It became apparent it was not going to work very well and we attempted to get relief from the new speed assignment. On the arrival; after RECRD; around TWNSN; I was descending through 11;700 FT. The DEN Approach Controller changed at this time and we were given vectors off the arrival; a clearance to 11;000 FT and told to slow to 210 KIAS. In hindsight; I am unsure if I descended prior to TWNSN; which has a 12;000 FT minimum crossing restriction. The new Controller did not indicate recognition of an altitude deviation. We landed uneventfully from a visual approach to [Runway] 35L. After the flight; I discussed the VNAV disconnect with the Captain and my concern that I may have descended early prior to TWNSN. He was certain we had complied with all altitude restrictions on the arrival. I may be the only one questioning this deviation. Maybe there wasn't an altitude deviation?  DEN ATC needs to understand that safety can be compromised very quickly by switching back and forth among the arrivals. This is not the first time I have experienced these last-minute clearance changes with DEN ATC. Since the arrivals (JAGGR and PURRL) are so closely aligned; ATC should consider not changing arrival clearances once an aircraft has commenced the arrival. They could provide the same end result by informing the crews of the new landing runways; clear them to cross specified fixes at appropriate altitudes; and eventually vector them off the arrival for an approach. In this case on the JAGGR Arrival somewhere around HASHE or HARUM would be good fixes to begin vectoring aircraft. This would have provided the same type feed for a north flow as the PURRL with less distraction and potential for mistake. As a crew; when things starting getting out of hand with the quick changes in clearances; we could have declined that last clearance and requested altitudes and headings from that point; decreasing the chances of mistakes from multi-tasking brought on by late ATC clearances.


Second narrative: [Report narrative contained no additional information.]


Synopsis: After ATC revised a B737-700 RNAV STAR from the JAGGR to the PURRL; then to the JAGGR and finally back the PURRL the flight was unable to meet the descent crossing restrictions. ATC then vectored the flight; slowed it and issued a descent clearance. ATC does not understand the consequences of such clearances.