You cannot buy up to PE using Airpoints. This can only be done by cash. Once you pay the fare difference, your ticket will be re-issued and you will be in a paid PE fare, earning PE mileage. You can then use your Silver upgrade to Business. If successful, you will still earn mileage at the PE rate. Note if your upgrade is cleared, they will re-book you to Business booking class R which is an ineligible class to earn partner miles. If your flight doesn't credit automatically, as it often doesn't in this case, you may have to ask TK to credit it retrospectively with some convincing to do.