PPL or CPL you still need to demonstrate aircraft handling skills and a depth of knowledge that is much higher than required to pass the PPL course.
Even experienced PPL's are going to to need a good brush up on seldom used skills and forgotten theory and a newer PPL is going to need considerable work to get to the minimum standard to start the FI course. The bottom line is PPL to FI is certainly doable but it will not be cheap or easy and will require a lot more work than most people think.
Remember being an FI is teaching a skill where the penalty for getting it really wrong is injury or even death.....