.. or is there an option to export all data from the database without database itself, then I can change the ddl, create a clean database and import the data again? If I do it manually, I must show in every table if is there a relation in another one.

I guess the best solution is either to extract the metadata (isql -x) or use gbak restore only the metadata, make the changes to a new, empty database, then use one of the load utilities that can be found at ibphoenix.com under download/tools/migration.