OK, now we're in the right place I can make some comment!
Virgin has had this experience with aircrew trained on the A340 who then moved to the B747-200 on promotion to command. The problems were significant. Regaining spatial and situational awareness using the limited aids and, more significantly, displays of the Classic was very difficult for those transitioning from the A340. This was, as I saw it (ex-mil trainer and QFI, but new to Virgin at the time), a more serious problem either than regaining the basic instrument scan, or than flying an aeroplane with few built-in protections against mishandling.
There was no easy answer to the problem other than more hours - mainly in the simulator. Some good pilots failed to successfully master the old technology, however.