Hi, I'm Eduardo Alvarez, I'm an Electrical Engineer with more than 6 years of experience in the design of electrical power systems for low voltage industrial, commercial and residential buildings.
Checking out the information you provided, yes, it's correct you will need to add a step down transformer in order to reduce the rated output voltage of 480V to 208V, if you want or need to feed a panel board with 120V/208V loads (1Phase or 3Phase).
Quickly taking a look at the file that has the electrical plan with sheet number PV.4, I assume the step down transformer would need to be located somewhere a long the line at the output of the 800A circuit breaker with ID "(E) TYP" installed in the main switchboard, "(E) Facility Loads". I would need all the information regarding the electrical loads to be serviced downstream of the switchboard in order to do a load study and define the dry type transformer to be used and calculate the feeders and OCPDs or to add a new panelboard to tie to the existing system.
I will use Etap Powerstation software to do all the calculations and simulations of the electrical system.
According to your requirements, I can assume your project with complete confidence and commitment. However I would like to discuss more about it in the chat, if that's OK.
Hoping to work together.
Best Regards.