WE HAVE AN EXISTING 3 PHASE 120/208Y MDP WITH A 700A MAIN CB. THE SERVICE ENTRANCE WIRES ARE PARALLEL 400 KCMIL.WE HAVE BEEN UNABLE TO FIND A TRANSFER SWITCH RATED FOR 700A AND ARE CONSIDERING USING AN 800A RATED ONE WITH A 800A MCCB THAT HAS ADJUSTABLE TRIP SETTINGS. IF THE ADJUSTMENTS ARE COVERED AND NOT ACCESSIBLE, WOULD THIS BE ACCEPTABLE?

If you meet the requirements of 240.6(c)...yes.

Some manufacturers make breaker plugs that you change out to set the ampacity fixed.