Will the call to the new function always be preceded by a call to the existing function?
If so then you can reverse the process by storing a list of locations where you removed spaces from.

Otherwise Shabbir's correct; if you're just writing a new program to get "C is the..." out of "Cisthephilosophyoflife." then you need to know somehow that C, is, the etc are words that would normally be followed by spaces, and this is further complicated by the fact that "I" is a word, so you could end up well confused if you start decoding the string as "C I ". So you will need some way of determining how long a word is. Also you'd need to make sure that "life" isn't followed by a space.