My guess is that it's nothing that can't be modified it's more of a case of whether it's a job that needs a pro or just someone competent. I don't believe it's a straight swap as I'm fairly sure the mounting points are different but there are guys out there who specialise in such mods that I reckon would modify any seat to fit a purpose.
One thing I would be careful of though if your doing it to ease your back pain is that you make sure the sitting position between the Dunnydore and the Nav are similar after all it might not actually be the seat that makes the difference in the Dunnydore it might be the position and how your legs and back are placed in the seat.
For me the sitting position between my Nav and the old mans Crewwie is quite different in that it feels like I'm sitting closer to the floor and my legs are straighter in the Crewwie as opposed to the Nav where it appears the knees are higher, whether thats exactly how it measures isn't something I've checked but it is something I'd have to consider before a seat transplant for the relief of pain as opposed to aesthetic reasons.
