Use logical indexing:
A = rand(10,10, 10) - 0.5; % contains random numbers in [-0.5, 0.5]
B = zeros(10, 10, 10); % preallocating the new matrix.
Aneg = A < 0;
Apos = A > 0; % Aneg and Apos are 10x10x10 logical matrices
B(Apos) = A(Apos) * (-1000); % which you can use to index into an array
B(Aneg) = A(Aneg) * 100;
histogram(B(:))
