Hi Vinay,
Hope you are doing good.
Well, in case of migration, you might have to compile all the custom task flows with 11.1.1.8 and create a new jar(Shared Lib) file to deploy them. Most of this will be taken care by Jdev and will be automatic migration.
As far as resources like page templates, pages , catalogs are concerned, there should not be any challenge.
Below is the link to Webcenter Portal Upgrade Guide
Though this links talks about migrating from Oracle WebCenter 10.1.3.x applications, but pointers mentioned in this are crisp and useful.
Just a question : Which and where are you using APIs in your WCP application?
Hope this was useful.
Stay in touch :)
Thanks
Rohan