RVSM
The 2004 date refers to December 2004. And it is not noise related. Stage 3 noise requirements exempt aircraft below 79,000 pounds. Stage 4 may change that, however. Some airports have restrictions on Stage 0, 1 and 2 aircraft. TEB, SMO and Naples, FL come to mind.
The real reason is RVSM. Reduced Vertical Separation Minimums. That goes into effect December 2004 and effects all aircraft.flying at or above FL290. There are two companies working on a mod for the LR20's but no one has come up with a firm price.
There has been a $200,000 ballpark price thrown out there. Yeah, it sounds expensive when compared to $400,000 to $800,000 for the aircraft. But when you realize what it will cost you to replace it, it becomes acceptable. From what I have been told about the mod, it will only be applicable to the later aircraft, having the Century III with soft flite wing.