Exisitng mechanical systems for renovations question

I am a little confused on the code for this. I have spoke with a few peers and not everyone can agree. We are working on an TI Alteration project. The architect is changing the occupancy of the TI from retail to office space. The Core and shell mechanical system built in the 1980's has DX AC units serving every level. Its capacities are still large enough to handle the cooling, heating and ventilation requirements of the new occupancy per 2015 code load and ventilation calculations. Almost all ductwork will be demolished and replaced. Codes: 2015 IBC & IMC (seattle amendments), 2015 SEATTLE ENERGY CODE.

Question
1) Can the existing AC units remain from a code stand point(never mind the life expectancy and age part of it)? My understanding of the code is yes. We are not replacing it and we are allowed to keep it as it.
2) If we were to replace the aging AC units in our design for the same tonnage, do we need to meet the latest Seattle energy code efficiencies?
3) Since the classification is now office and we have some high density spaces, do we need to adhere to the 2015 code and provide demand control ventilation or since the building was already permitted without it, we can omit in our design.

The code seems unclear to me for this situation. Any thoughts or comments are appreciated.

Thanks