What is the best way to convert a large site using 'px' to use 'em'/'rem'

The subject pretty much says it.

I've got a large software application using web technology with a large css file and some in element styles (yea I know).

I guess I'm hoping for a formula or algorithm so I can write a little app that will convert all the px values to em or rem, based on px value.
Also, what css properties should I exclude?