You can just end them and choose "Sell similar" from your ended item list and list them as fixed price from that.
Just want to stress again that book buyers for common books are going to almost universally search price, lowest first select the cheapest one that matches their buying criteria and most will ignore auctions and will not care about your return policy. Ebay wants you to offer free returns because it is not their money and thus they rank that higher in search, however that ranking change vanishes when the buyer searches lowest price first. Most book sellers do not do free returns. A quick search of "The Shining", which is a common book that sells multiple times a day shows that in recent history 1000+ copies sold and only 172 of them were from "Free Returns" sellers.
Return policy is much more important in rankings for clothing, in which it can be nearly impossible for buyers to actually compare a bunch of identical items side by side and actually shop based on price, so search ranking matters a lot more with clothing.
You are going to have a wide number of opinions on return policy. It is definitely possible to thrive as a seller with any return policy that you choose. However in all cases you absolutely want to minimize returns and I suggest blocking anyone who ever actually returns anything since one return can suck the entire profit out of many sales, particularly with cheaper, heavier items.
