There is a very trivial way to move users objects from one TS to another with EXP/IMP.

1. Perform an EXP
2. Drop user togerher with its objects
3. Racreate the user with new default tablespace
4. THIS IS THE IMPORTANT PART: revoke user any quota on any tablespace except for the new default tablespace. Pay also attention that user haven't got UNLIMITED TABLESPACE system privilege (it used to come together with RESOURCE or CONNECT role by default)
5. IMP the dmp.

Oracle will try to create the objects in their old tablespaces as it is recorded in the dmp. But since the new user has no quota on the old tablespace the objects will be created in new default tablespace.

Bottom line: it is not sufficient to define new default tablespace for schema owner, you must also rewoke him quota from old tablespace.

HTH,