You cannot convert preset bonuses... but you can add bonuses onto a piece when there are none. I'll use your piece as an example; If you had rolled a 5% FF only with no +2 vision, then after you purchase the piece you could then add +3 speed. Also works the other way (if you had rolled a +3 speed only piece, you could then add the 5% FF). Best to target one OR the other bonuses when rolling for AEQ and count on adding what you want later. Count yourself lucky to roll exactly what you're looking for. And be aware of how many times you "refresh" when shopping. Sometimes you can spend more just refreshing than its worth in the overall cost of the piece you're looking for.