How feasible is to change a C/C++ cgi application to Fastcgi? Does this require only change in code? Or will it require a change in the setup of apache server?
What will be the obvious benefits of the change? Is the change from cgi to Fastcgi worth the benefits?
The major benefit of switching to FastCGI is that your application will run continuously in the background, instead of being started for each request. If your app has a lot of initialization, you could see a big speed boost by cutting out that time.
It will require reconfiguring Apache, but it's not complex. And it should require changing your app, because now there's only one instance per server, not one instance per request. You'll have to decide for yourself if that change is feasible; it really depends on how your code is structured.