Question: Change of variables - Convolution integrals

Hi, I'm new to Maple and I'm trying to use it in some frequency domain convolutions analysis. At the moment, I'm just tying to organize my work and for this I need to perform some simple variable changes on multiple integrals.

I've started like this: 

> conv:= (x,h,t,tau) -> unapply(int(x(tau)*h(t-tau),tau),t);

> Y[1]:=omega->H(omega)*X(omega);

> with(IntegrationTools):

> Y[2]:=conv(Y[1],Y[1],omega,sigma1);

> Y[3]:=conv(Y[2],Y[1],omega,sigma2);

Now, what I really wanted to do is something like this:

> Change(Y[3](omega),{sigma1=z1,sigma2=z1+z2});

The result looks like the way I'm expecting. However, I've tried to do this:

> Y[5]:=conv(Y[2],Y[3],omega,sigma5);

and tried to do two simple change of variables, just to see if everything was ok:

> Change(Y[5](omega),{sigma1=z1,sigma2=z1+z2});

but got this error: 

Error, (in IntegrationTools:-Change) invalid input: rhs received sigma2, which is not valid for its 1st argument, expr

I've looked at the Maple Help and the internet, but wasn't very sucessful. Anyone could give me a hand?