In a quick overview:
Upgrade the Designer 6i repository to the Designer 10gR2 repository, Designer 11g does not exist.
Remove the obsolete code for the client/server. Client/server code should be rewritten.
Update your headstart to the latest version for Designer and your database.
Update headstart code inside the application specific library. (hopefully your Headstart library were not changed)
Rebuild your capi/tapi
Rebuild your forms/liibrary/menu
Do a lot of testing, to see if everything still works.