This is a part of the apex documentation but the fact is the most of us simply don't know about it. I have several customers and those customers have different versions of Apex installed. I am using XE on my Laptop to develop their applications. Therefore I need to be able to use different versions of apex simultaneously. With a help of the two simple scripts wrapped into an executable, I can switch between the versions in seconds:
ALTER SESSION SET CURRENT_SCHEMA = FLOWS_030000;
to switch to the version 3.0.1 and
ALTER SESSION SET CURRENT_SCHEMA = APEX_030200;
to go back to the latest release 3.2.