As a Brit, if you refare, you get the obc offer on the new, lower fare, assuming there is one. So you lose any previous obc.
Therefore, you really need to do the maths before going for the refare option, especially as you can only do it once.
For our forthcoming April cruise, we refared to a minisuite. Lost the obc on our standard balcony cabin, but got a better cabin for the same price even taking the £100 "penalty" into account.
And our shareholder obc compensates for the lost obc, which we probably would only have spent because we had it.