If you want to produce an indication of how changes in CAS or Mach number tend to affect the crossover altitude you can do so with a simple sketch.
Draw a vertical and horizontal axes for a graph. The vertical axis represents altitude increasing from bottom to top, and the horizontal axis represents CAS increasing from left to right.
Draw a vertical line from bottom to top at about the middle of the horizontal axis. This line represents a constant CAS climb.
Now to draw the CAS equating to any constant Mach number we must remember that the speed of sound as a CAS decreases as altitude increases. So to represent a constant Mach climb we must draw a line sloping downwards from left to right.
The point at which the two lines cross is the crossover altitude.
Now label the lines with approriate values, let's say 260 Kts CAS and Mach 0.74.
To see how increasing CAS changes the crossover altitude, simply draw another vertical line to the right of the first. Let's say we have chosen this to represent 340 Kt CAS. You will see that this higher CAS line crosses the Mach 0.74 line at a lower altitude. So increasing the chosen CAS decreases the crossover altitude.
Now draw another sloping line (to the right of the original) to represent a higher mach number. Let's say Mach 0.78. You will find that this crosses the original CAS line at a higher altitude. So increasing the chosen mach number increases the crossover altitude.
This exercise indicates that increasing the chosen CAS will decrease the crossover altitude, but increasing the chosen Mach number will increase it. If of course we increase both the CAS and the Mach number then the change in crossover altitude will be the combined effect of both changes.