Astro14 and 757guy gave you a pretty comprehensive description of what it was/is like to transition between aircraft in the airlines as your "Permanent Position" changes (B757 Capt to A330 Capt, A320 FO to B757 FO, etc). The training footprint, as someone noted, is like a two month long hostage crisis. Do it in Moscow on the Mississippi (MSP) in January and February and it felt like a Soviet gulag.
There were those of us at NWA (Not Written Anywhere, Not Without Acrimony,etc) who were "Forced Temporary" pilots, who transitioned between different airplanes and seat positions EVERY month OR sometimes between different airplanes and seat positions in the SAME month. It was done in reverse seniority order. Each month, as the airline flexed its flying operations up or down, if you were the most junior pilot qualified in that position, you got tagged for involuntary temporary reassignment. I was double/triple qualed from the Fall of 1990 until our contract changed after the lockout/strike of 1998. It was the worst period of my 25 year airline career and I was burned out. I was constantly taking checkrides, recencies and OE almost monthly, at least it seemed that way. Further, since you were already "qualified" on the aircraft you were in the simulator for, you were expected to perform quickly and precisely, notwitstanding the fact that you had more recently flown up to two different aircraft besides this one.
From month to month, I might have to look two or three different places to find out what aircraft and seat position that I was flying next month. As the next month rolled around, a week prior, I would start getting out my flight manuals for the "new" airplane and start rememorizing aircraft operations limits, schematics of hydraulic, fuel, electrical systems, pressurization, engines, ad infinitum. I would have to completely relearn aircraft approach procedures and instrument landing callouts/procedures for the new jet, even while flying my current seat position during airline trips. It was routine for me to take two sets of flight pubs on an airline trip and study on my layovers. I might land on the last day of the month as an international 747-400 FO and walk out the door a couple of days later as an A-320 Capt starting a domestic trip. In Dec 1993, I walked in the door after flying an Asia trip as a 400 FO and the phone was ringing. I was being "forced temp" to B727 Capt the rest of the month, while already being scheduled for an A-320 Capt Checkride after Christmas. That would have been three different aircraft and seat positions in the same month. I told the Crew Scheduler what they could do with their request since I was supposed to have a few weeks off over the Christmas holidays. Expecting a call from the DTW Chief Pilot at any moment for my unenthusiastic response to Crew Skeds, it never came. Perhaps the Chief Pilot was not such a Scrooge after all.
Exacerbating this dual/triple qual scenario in the airline, I was also flying in the Air National Guard. The Guard airplane made my 4th current airplane at one time. I would simply do what I did in the airline- review ops limits, systems, ejection and boldface emergency procedures before each flight and hope that I could use my superior judgment and not my superior flying skill. I finally used my superior judgment and left the Guard so I could focus on exclusively airline flying.
NWA was the only major US airline-or any airline that I know of- that employed dual/triple qualified pilots. I spoke with ALPA, NWA and finally the FAA about it. They all said the same thing- "we feel your pain, but it is legal." If the flying public had only known...
In retrospect, changing aircraft positions in the airline was actually both challenging and something to look forward to. It might mean being able to now fly international routes on a widebody when you had only flown domestically. It might mean you get to upgrade to Capt from FO. It might mean upgrading from "steam gauges" to the latest "glass cockpit" technology with a whole new language of acronyms like EICAS, ECAM, FADEC, IRS/GPS, MCDU/CDU, MCP,PFD/ND, FMS, etc. It was definitely the "firehose" effect when you went to school-particularly on the "glass" jets. The learning curve was exceedingly steep but our Instructor Pilots (Capt and FO) were generally outstanding (guys like Astro14, 757guy, etc). US airline training programs are outstanding in their scope and methods and produce excellent pilots. A few of us marginal guys did manage to slip by though...maybe that's why I got all those extra checkrides and line checks