2000 amp Service, 3 phase 4 wire in HI RISE Building.

Hello thinkers, I have a question for you! How do I retrofit a 2000 amp service without losing power to the 250 units it serves. I need to limit my down time to less than 24 hour periods. The current service is 35 years old and showing signs of wear and maintenance problems for the maintenance staff. It is a 1600 amp bus duct with 29 switches to disconnect feeders going up the riser area. Each feeder takes care of about 8 units.

I will reward the right answer with the design contract or a fee for their help if I get the job.