Yes it would most likely require some tooth substance to be removed. Amalgam fillings are retained in a cavity by undercuts in the cavity walls. An inlay is not retained by undercuts, and the undercut areas of the amalgam cavities would have to be modified by removing some of the tooth, to allow the inlay to be seated in the cavity. In most cases, the amount of tooth tissue to be removed would be small, providing, as you say there is no decay etc.