Older version of gfortran doesn't get installed despite removal newer version I installed gfortran v6.3.0 using installer, and needed to go back to 4.9.2.
I removed gfortran using sudo rm -r /usr/local/gfortran /usr/local/bin/gfortran, but when I try to install v4.9.2 using a disk image, it says a newer version already exists. 

How do I resolve this issue?
You can use apps such as App Cleaner to fully uninstall apps. 

In your case there are likely files left on your computer for v6.3.0 and using App Cleaner will remove these additional files.

You can do this manually by finding and deleting the .plist and .bom files associated with gfortran. Type 'locate gfortran' in a terminal window will find the path names to these files.