Hi Dick,
If it was my CJ, I think I would wait a few years for the price to come down.
I don't know how much time you have spent studying the specific fuel consumption of your jet at optimum cruise altitude vs FL280. As a reasonably wealthy man, I'm just speculating you may not have bothered in the past.
But if you do, you might find that cruising at max range speed vs max speed at FL280 gives you a much lower flight fuel. You might even find it's only marginally more than cruising at optimum altitude. Yes it will take longer to get there.
If you don't fly often, the increased fuel burn plus lower future upgrade cost may more than offset the cost of upgrading now. Would be interested in your figures if you care to calculate and post them.
For what's worth, I am a narrow body domestic airline pilot, not a private jet pilot, but working for an airline mandates a high awareness of fuel vs schedule decisions when cruising at lower levels. I often cruise at lower levels for headwind or turbulence reasons, and at lower levels your cruising mach/IAS makes a big difference to fuel burn.
Keep up the dedication to the cause. Fred.