If you know C++ in depth, you aleady know C at a professionally usable level, or at least get there with a couple of projects. C is a subset of C++ anyway. True, there are minor differences hera and there, but shouldn't really be deadly.
I guess the more difficult part will be to transition from object oriented to procedural paradigm.
I know few things such as structs etc. and how you can derive a class from a struct and stuff. but i have never used calloc or malloc (shudder!) and have only relied on new/delete []
for all memory management.
Thus its hard for me to understand what constitutes as C and what dosen't. I mean i dont know if C supports typedef's
I guess the more difficult part will be to transition from object oriented to procedural paradigm.