Take a look at the file seam21migration.txt that is in the distribution. Is described the needed steps.
Well, yes, I read the file.
It mentions the required changes to the application code but as I said, my application is generated via seam-gen with only two entities (I didn't code any thing).
Is the only thing I have to do is rerun seam-gen tool and copy Seam 2.1 libs to my application libs dir?