Reworking paintings has been a common practice throughout art history, with artists modifying their own work at various stages and occasionally altering pieces by others. Advances in chemical imaging and increased access to traditional imaging techniques have facilitated the documentation of such interventions. Initially focused on Old Masters, research on reworking practices has expanded to nineteenth- and twentieth-century artists. For the first time, a classification system for reworkings is introduced, based on the oeuvre of Belgian Modernist painter James Ensor (1860–1949). Five representative case studies each illustrate one of the proposed types of reworking: (1) pentimenti, (2) post-factum revisions, (3) recycled works, (4) metamorphoses, and (5) appropriations. Using advanced imaging and spectroscopic techniques, including Macro X-Ray Fluorescence (MA-XRF) and instrumentation from the Iperion HS consortium, the study also provides material-technical evidence of Ensor’s experimental studio practice while shedding new light on anachronisms in his oeuvre.