Once an umpire declares a foul ball can it be overturned?

Depending on the call all umpires can reverse a call but the crew chief supersedes all umpire ...

Clarification:

The chief umpire for the game, aka. plate umpire makes the final decision on a call when another umpire ask for help. If the umpire does not ask for help no other umpire should interfere with the call. The home plate umpire is the chief umpire, unless the league appoints an umpire as the chief umpire -- MLB appoints a crew chief for every umpire crew, all calls where umpires ask for help will come down to his final judgement. You can find all of this in 9.02 and 9.04 of the rules