MPF has got nothing to do with Permanent residency.
The oath confirms that your leaving HK and want to cash in your pension and that if u do decide to return then u cannot withdraw it again until retirement.
But someone on this forum said its always good to leave a bit in you MPF account, saves the hassle in case you come back. Maybe getting a copy of the oath in advance would help.
That would be really funny though, on the one hand you take an oath that your permanently leaving HK and u can have your MPF money back and then at the airport HK Immigration asks you to surrender your permanent ID card.