In the first place the aircraft has to have a certain navigation accuracy to keep the aircraft operational.
That's the main driver for this modification.
With the old navigation equipment this wasn't possible anymore. The photo of the "old cockpit clearly showed that the original Delco INS navigation was still present. These old INS systems are also a maintenance nightmare in the present time. Less and less possibilities for repairs of this mechanical/analogue equiment are available in the near future.
The integration of a FMS , with a less accurate "strap-down" IRS but updated by GPS in an analogue built aircraft is already certified by several STC's on many 747 classics and is relative cheap.
IMHO the only "new"part of this upgrade is the size of the LCD
screens and the fact that for the first time a (part of) the old dial instruments at the F/E panel are replaced by LCD
This part of the modification is to lower the maintenance costs, because the analogue instruments will become very difficult to be serviced (=very expensive) in the near future.
Clearly this cockpit modification was the most cost-effective option to stay operational and satisfy the present and future navigational requirements and at the same time to lower the maintenance costs.
Modifying a more recent digital built aircraft to "Sofia-standard" would be far more expensive and funding for that project almost impossible.
[Edited 2012-12-09 02:31:20]
Operating a twin over the ocean, you're always one engine failure from a total emergency.