Pl identify exact versions of "9i" and "11g" - review the New Features guides for each release above "9i", as they document delta changes.
10gR1 - Contents
10gR2 - Contents
11gR1 - Contents
11gR2 - Contents
For the most part, you will have to make few, if any, changes, assuming good coding practices were followed (again assuming this is a custom developed software - if it is off-the-shelf, the vendor will certify the software against various database versions).
Best to perform a test upgrade and evaluate what, if any , changes need to be made to the application
Welcome to Oracle communities.
Also please mention if the migration mean just an upgrade from 9i to 11g on same box or to a different new box.
Whether the platform is same or different like linux to linux or between 2 different OS etc.,.
As mentioned you need to check your application compatibility with new version. Also upgrading/migrating data have many options like direct upgrade using DBUA or Manual method if same system.
For migrating to other server export/import or OS copy if same OS etc.,.