Is it OK to Switching Undo Tablespaces without DB stop ?

Question

We use Oracle11g R2.
I read Oracle Doc and It looks OK.
But, how could I make sure UNDO tablespace mode(ONLINE,PENDING OFFLINE) ?
Switching Undo Tablespaces
You can switch from using one undo tablespace to another. Because the UNDO_TABLESPACE initialization parameter is a dynamic parameter, the ALTER SYSTEM SET statement can be used to assign a new undo tablespace.
The following statement switches to a new undo tablespace:
ALTER SYSTEM SET UNDO_TABLESPACE = undotbs_02; 
Assuming undotbs_01 is the current undo tablespace, after this command successfully executes, the instance uses undotbs_02 in place of undotbs_01 as its undo tablespace.
If any of the following conditions exist for the tablespace being switched to, an error is reported and no switching occurs:
  The tablespace does not exist
  The tablespace is not an undo tablespace
  The tablespace is already being used by another instance (in an Oracle RAC environment only)
The database is online while the switch operation is performed, and user transactions can be executed while this command is being executed. When the switch operation completes successfully, all transactions started after the switch operation began are assigned to transaction tables in the new undo tablespace.
The switch operation does not wait for transactions in the old undo tablespace to commit. If there are any pending transactions in the old undo tablespace, the old undo tablespace enters into a PENDING OFFLINE mode (status). In this mode, existing transactions can continue to execute, but undo records for new user transactions cannot be stored in this undo tablespace.
An undo tablespace can exist in this PENDING OFFLINE mode, even after the switch operation completes successfully. A PENDING OFFLINE undo tablespace cannot be used by another instance, nor can it be dropped. Eventually, after all active transactions have committed, the undo tablespace automatically goes from the PENDING OFFLINE mode to the OFFLINE mode. From then on, the undo tablespace is available for other instances (in an Oracle Real Application Cluster environment).

Answer 1

After you switch the UNDO_TABLESPACE parameter to the new tablespace, at the minimum, you have to wait for active transactions to complete (commit/rollback) before you can take the old Undo Tablespace offline.  Note that if you take the old Undo Tablespace  offline too quickly, you may prevent long running queries from reading from the old Undo Tablespace and may cause them to error with ORA-01555.  So you need to wait a reasonable time (e.g look at MAXQUERYLEN in V$UNDOSTAT) before you take the old Undo Tablespace offline.
Hemant K Chitale