Since the System Classification will normally be greyed out, switching directly is not possible. You can try one of the two possible ways to switch to the correct system classification:
a. Disconnect the pipe from the pipe fitting which belongs to a different system
b. and then reconnect by stretching the pipe back to the fitting (either we can use trim method for re connection).
a. The second method is, press the “Tab” key and select the fitting, this makes the system highlighted in the System Browser (System Browser opens from View tab -> User Interface -> System Browser or by pressing keyboard shortcut “F9”).
b. Once selected, press the Delete key which basically deletes the System. Then reconnect the pipes back to the fitting.