I was able to downgrade from 8.1.5 to 8.0.5 and able to bring up the database.

But when I tried "sqlplus system/manager"
I am getting following error..

Error accessing package DBMS_APPLICATION_INFO
ORA-00604: error occurred at recursive SQL level 1
ORA-01653: unable to extend table SYS.IDL_UB1$ by 15 in tablespace SYSTEM

then SQL>

Please let me know how can I resolve this problem.