Since when can you not convert this way? I believe it's just YMMV as some stores allow it while others don't.
It came up when I was trying to buy some eShop cards online with BB GCs. Gave an error I've never seen on BB before about "cannot purchase gift cards with gift cards." Tested it on a regular BB GC online, too, and it got the same error. This was a few weeks ago. Haven't tried it in store, yet, but figured that since it stopped working online, it's probably stopped in-store, too. Has anyone purchased GCs with GCs in-store in the last few weeks that you know of?