How to change the variables of a function?

How can I transform: u(x,y,z) to u(x0,y0,zR)

Where: z = zR + f;

u (x, y, z) = 1/(I (Lambda) z)Integrate(u0 (x0, y0) Exp((I k)/(2 z ) ((x – x0)^2 + (y – y0)^2)), y0, {x0, -Infinity, +Infinity}) ;

u0 (x, y) = P(x, y) Exp((-I k)/(2 f ) (x^2 + y^2));