I was going to tell you that you needed a special key, then I read the entire post!!
I have an Ocean 600 as well and had no problems changing the barrel. I have also changed them on the roof bars as well so that as you say only one key is required.
I found that the roof bar locks came out very easily, whereas the box one needed some 'assistance', this I hasten to add was not by means of a huge hammer!!
Hope this helps