I have just checked one of my current listings and you can't change a 'buy it now' to an auction.
You will need to cancel all of the items you want to change, then relist them as auction items with a starting price to sell as auction items.
You will be able to find the cancelled listings in 'unsold' and won't have to start listings from scratch.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)