Hi Damian, it is not uncommon for the finish of furniture to change slightly from batch to batch. The differences will be almost impossible to distinguish at first but after 8 years all of these slight changes will compound and be quite noticeable. As a consequence, we wouldn't recommend trying to match units when there has been such a time gap.
At the moment, you can't but additional handles although this may change at some point in the future.
We don't make the Mix It Drawers and Wardrobes, they are sourced from a supplier in Italy exclusively for John Lewis so, unfortunately, don't have this information.
Sorry I wasn't able to help further, best wishes.