Can someone explain what the differences between an incompressible and compressible flow cfd code are. I have experience with an incompressible code, but would be interested to know how easy/difficult it would be to convert it to a compressible code.

A compressible code typically solves for velocity and two variables of state using equations derived from the conservation of mass, momentum and energy. An incompressible code assumes a constant in space, specified, thermodynamic pressure as one of the variables of state leaving a perturbation pressure, a state variable and velocity as the unknowns. Often isentropy is also assumed and the energy equation dropped.

How easy it is to convert an incompressible code depends on what is in the incompressible code and why you want compressibility. Some incompressible combustion codes are fairly general and will not need much modification but many constant density codes will need a lot of modification. Accurate, low Mach number compressible flow is difficult which is why incompressible codes were introduced. High Mach number compressible flow is easier but if you want accurate shocks and the like the numerical scheme will need changing quite a lot.