I went back today and they already had the old lock off. I picked out a new one (no key for me to lose, just a combination to remember) and presto-changeo I had it screwed into place 10 minutes and 2,000 yen later. Woohoo!
But the best news is I have a couch!!!
See that lamp on the floor? 900 yen. Oh, recycle shop, how I love you!