I believe that if you upgrade from Hilton Amex to Hilton Surpass Amex, the spend carries through. So if you'd spent $39,000 and upgraded, I believe that when you hit $40,000 you'd get Diamond.
I'm not sure what happens, though, if you've already exceeded it. If you know you will spend that much again, definitely get the Surpass or Citi Hilton Reserve.