Altitude limitations for the aircraft may be based not upon the ability of the diesel engine to operate well at high altitude, but to be restarted up there. This was the basis for the altitude limitation on an earlier diesel aircraft.
The opportunity to retrofit an SMA diesel in an older 182, though theoretically there, is practically not. I ordered an SMA engine, on behalf of a client, in March 2010, and the aircraft has been waiting firewall bare since then. The older SMA diesel, though STC'd in the 182, is no longer available. 47 were converted. The new SMA "E" engine is not STC'd for retrofit - the future will tell. I have met with the SMA President with respect to the purchase of an SMA engine on more than one occasion. SMA staff were very welcoming, and attempted to fulfill this order. Though things did not go the way I would have hoped, I do understand SMA's priorities with respect to the "aftermarket" use of their engines.
Cessna's SMA diesel 182 will be a remarkable aircraft.....