One way, but note there will be loops in the background:
X = [A.X];
Y = [A.Y];
Z = [A.Z];
[theta,phi,r] = cart2sph(X, Y, Z);
THETA = num2cell(rad2deg(theta));
PHI = num2cell(rad2deg(phi));
R = num2cell(r);
n = numel(A);
[B(1:n).theta] = deal(THETA{:});
[B(1:n).phi] = deal(PHI{:});
[B(1:n).r] = deal(R{:});
Also, note that keeping each individual number as its own field in a struct is not a very memory efficient way of storing your data. Unless you need the data in this format for some other reason, it would be better to simply keep the data as double arrays (e.g., like the X, Y, and Z arrays).
