Will I see and major handling changes?
Short answer: Depends.
Longer answer ...
If you put the same-width tyre on the 18" rim as is on the 19", because tyre height is a function of tyre width, the overall diameter will be 1" smaller, so the front will drop 1/2". This doesn't sound a lot but it does steepen the effective steering angle; this might
translate to more wobble-prone.
You can compensate for the smaller rim diameter with a fatter tyre; because tyre height is a function of tyre width, a wider tyre is also taller - e.g. if the 19" rim has a 100/90 tyre on it, if you put a 110/90 tyre on the 18" rim, the overall diameters will be very similar.
But increasing the tyre width makes for slower steering ...
Should I wait till I find a 19" spoke wheel.
As the KZ400 wheel is spoked, have you considered just rebuilding it with a 19" rim?