Is it permissible to change the intention of prayer from audible recitation to silent recitation while performing it, or vice versa?

If you pray a supererogatory prayer at night, you may recite aloud or silently, depending on what is more invigorating for you. If you are reciting aloud and someone enters, there is no harm in completing the recitation silently. You may also begin reciting silently and then switch to reciting aloud. As for supererogatory prayers during the day, the Sunnah is to recite silently, but there is no sin upon you if you recite aloud.