upgrade the camera body - alone? there are a few alterations that can be made to higher end DSLRs - mostly the replacement of the existing viewfinder for a focusing aid (though they do also mess up your AF and in camera metering as well - so there is a tradeoff) you can also get a few other things like LCD light covers (little covers that help block sunlight so you can view it clearly) and also different viewfinder eyepieces - anglefinders magnifiers. Aside from that other upgrades and additions are all external - remote releases, tripods, lenses, flash brackets, flashsetups.... DSLRs are nearly all totally sealed setups - the only fully upgradable DSLR system would be the RED modular camera system, its diverse and very flexable, but not that cheap
