change oracle user

1) check is your session in multuprocessing mod
select sys.kupp$proc. IS_MULTIPROCESS_ENABLED from dual;

2) If it is, disable moltiprocessing
select sys.kupp$proc.disable_multiprocess from dual;

3) switch current schema
alter session set current_schema = USERNAME;

4) change user

exec sys.kupp$proc.change_user('USERNAME'); 

more about this package at http://www.morganslibrary.com/reference/pkgs/kupp_proc.html