Migrating uVision project within the same controller family
These are the things that were required to migrate existing STM Keil project to another controller of the same family or of a different family. 

Firstly, the options for target is of use when a new project is being created in Keil. Changing a particular controller in "Options for Target" does no good in an existing project.

Basically one needs to change the startup file which is unique for a controller. 

Then if you're using any peripheral library files to communicate with the controller, then you need to make sure that the library file supports that particular controller. 

In my case, there was no code/ defines for STM32F071. 
So i created a new project in STM's CubeMx software and generated the startup and the appropriate peripheral library files and copied that files to my existing library file with a macro #if defined (STM32F071).

Also after some searching i came across STM32F0's library file which had all the setup codes for different F0 microcontrollers; all embedded into one header file, separated by #ifdefs.