After passing the vehicle in this situation, are you allowed to change the low beam lights into high beam lights?

Yes. After passing the oncoming vehicle, you can, and indeed should, switch the main beams back on. You are travelling on an unlit road outside a built-up area, where using main beams significantly improves visibility and safety. The obligation to switch to dipped beams to avoid dazzling the other driver ceased once the passing manoeuvre was complete.