On the other thread about diesel heaters there was some discussion about them not being able to use existing heating ducts to distribute heat to all parts of the van like the bathroom, front dinette etc because they overheat. However having talked to Bill (Fisherman) you can with the Truma diesel combi which he has. So I googled it and on the Truma support page it says you can.
It looks like the latest generation Combi Diesel is pretty much a drop in replacement for the Combi gas truma system. Well its a bit more complicated than that and needs doing by a professional but it 100% says it can use the existing heating outlets.
support.truma.com