Thanks DWFI,

Well it was not super straightforward.

I called Exec Club and could easily change to a higher class as long as I have enough miles. In this case I did not and as it would become a cash + miles booking was told that I would have to cancel and re-book two one ways with the agent but was discouraged to do so (i.e. the agent made it sound like a hassle and I was concerned in losing my return flight).

So in the end upgraded to Club as had enough miles for that. And as an extra bonus was not charged an offline fee (I am not gold).

J