Thank you maru.
I tried your method on my current project because as always, I had last minute changes of vey localized areas of the images :)
So I took care to save a dump EXR of my render.
I resumed the render from the EXR, stopped it, defined a render region for the modified area and rendered. This wotks locally, but if I render in DR the rest of the image outside the render region is re-rendered again. So I have no option but to render only on my workstation.
Also I had some lens effects applied and these don't get updated on the whole image neither.