I'm purchasing a house, built in 1950 of concrete masonry units, with concrete exterior window sills. One of many changes I'd like to make before moving in, is to install rigid foam insulation outside the CMU walls. Unfortunately, it has concrete window sills that would need to be extended just as far as the insulation. Can anyone suggest a better way to deal with this, than to remove and replace the existing sills?

If the region makes a difference, the house is in Southeast Michigan. If this would be better posted in another section, please suggest which section.