I had the same problem when I converted my 440's to 20mm. There is probably a specific tool for it but I couldn't find mention of it anywhere on the DT swiss site!!
I ended up using some wooden dowel which suited the hole diameter inside the hub (had some slightly oversize so sanded it down which gave a nice tight fit).
Once this was inside the hub I screwed a 8mm self tapping screw into the dowel (end with the adapter still on, other end is acting as a handle to hold onto) which expanded the wood inside the hub and gripped it.
Hey presto, 15mm adapter screwing off!!
Was a pretty ghetto way of doing it but got me riding the same day!
Good luck with it and let us know if you discover the tool/ textbook method of doing the job!