what happens If I change the phase and magnitude of a signal
Regarding the phase: if you set all frequency components to have zero phase, then the signal becomes essentially a sum of sinusoids that tends to an impulse as the frequency increases. In other words,

$$\delta(t) = \int_{-\infty}^\infty \cos(2\pi ft) df.$$

In your case, the maximum frequency is limited, which means that the signal will actually become a sinc. Try this in Matlab (or similar):

fs = 1000;
t = -0.1:1/fs:0.1;
f = 0:200;

s = zeros(1,length(t));
for ff = f
    s = s + cos(2*pi*ff*t);
end
plot(s);


and you'll get a plot like this:


When each cosine has a different amplitude, the signal still will tend to acquire a sinc-like shape.