Selecting reinstall will not change any values you have applied to your installation, and will not remove/delete content.
The reason that configure existing installation is not available is most likely because a patch has been applied to the original version of the application. A change in a jar file can trigger the installer to not recognize the 'version' as being the same, therefore the only option is a reinstall.
If this is the scenario you are facing, make sure that you have any additional patches that you have applied on hand so that they can be reapplied.
Both reinstall and configure existing installation will always read the oasisconfig.properties file for the current application values and will not change any of this information unless you change it in the installation dialog screens.
Neither of these options will delete any content or modify the behavior of your processes.