Transmission lines with same impedance but different dimensions

In theory, there is no reflection at the connection of both TX lines.

In practice you would need to connect the outer conductor of the bigger TX line with the outer conductor of the smaller one.

This is where you might get an impedance change and thus reflections.

If you connect two transmission lines of different dimensions together, even if they are the same impedance, the physical discontinuity in the inner and outer conductors introduces an additional shunt capacitive loading, which does create a reflection.

This is usually combatted by what is called a Kraus (spelling?) step. In coaxial lines, this means staggering the locations of inner and outer diameter change, so that there is effectively a short length of inductive (higher impedance) line between the two shunt capacitances. This creates a low pass filter which eliminates the reflection, up to a certain frequency.

I take what you're saying to mean that the formula I gave for the impedance is not accurate near the interface between the two transmission lines. Is that correct?

Modifying the coaxial impedance formula 'near to the interface' is not a useful way to handle it. You could probably fit any one instance by doing it, but it lacks predictive power and simplicity. Keep the coaxial impedance formula as is, it is exact after all, and handle the non-uniform part separately.

The way all microwave engineers, and microwave tools like ADS and QUCS handle it, is to lump the effects of the step into an S-parameter box, that's connected between the two lines. A simple model would be a small shunt capacitor. More complex models exist which fit the exact results better.

This 'extra component' approach can be used whenever a transmission line comes to a discontinuity. The fringing capacitance and the radiation loss of an open end, the extra capacitance of a bend, or the connection of a stub to an otherwise uniform line, are all handled by having perfect transmission lines connected to a model of the discontinuity.

There are several ways to arrive at that model. Where the junction has some nice symmetries, it's possible to use transformations to arrive at the extra capacitance analytically. However usually either test pieces are measured, or 3D models made in something like HFSS, and the parameters of the discontinuity extracted from the overall results.