As part of an effort to make Concord more affordable and diverse, local officials want to shift the tax burden off most property owners and onto the town’s wealthiest residents. The long-shot proposal, which some say is unconstitutional, calls for imposing a local income tax to help offset the town’s dependence on property taxes to fund its operations. “It’s not a tax increase, but a shifting of the burden,’ [snip] “If we’re lucky, this could happen in 15 to 20 years. But if you don’t start somewhere, it would never ever happen.’’