I often have to change only the material on certain surfaces in the scene and I then render the whole image, this is very time consuming and a waste of pixels. 
for example I have an interior and a cupboard. the customer want to see the cupboards doors in different materials…
I now change the materials and send the whole scene over BB to the slaves to render. but is this necessary, I mean a renderelement would be nice. but there are some problems with the GI and its calculation.
how do you make similar things? In fryrender you can do that with SWAP.
thanks for your ideas…
best regards
themaxxer
I do a color wire element, which allows me to change colors in Photoshop, but materials might be a little more challenging.
we render only parts of the scene new, for example if the cladding of one facade is required to be shown in a few variations we render only roughly this part. then we past those elements into the master photoshop-file which works quite well most of the time, best of course if you have multimattes for the materials or objects.
this way we only have to tweak and post produce the whole image once and then paste in masked layers with the variations on them. but of course, if you have a shelf which is white and glossy and something green reflects in it and you change this said green element to blue you would have to alter those reflections as well. can probably be more time consuming than rendering the whole image a few times… i guess it depends.
and, if it´s simple we only color correct a masked part of the scene (via multimatte per material), if you have 16 or 32 bit images you can tune those parts very well in photoshop to any desired color without even rerendering anything… this works quite well for smaller stuff or bigger more uniform surfaces in our work.
but of course, you probably knew all that before maxxer, no revolutionary techniques on my side. 
cheers,
christoph.
CTRL + A, Deselect Elements u want to re-rerender > proporties > Untick Visible to Camera > Change ur object material - done…