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Many Ayurvedic drugs use roots, root bark, rhizomes or heartwood, parts whose harvest kills the plant or seriously damages it. Plant-part substitution proposes replacing these parts with renewable ones, such as leaves, young stems or small branches, from the same species. This review appraises comparative studies of plant-part substitution for Ayurvedic drugs and identifies what evidence is needed before a substitution can be adopted. Six studies were examined. For Premna latifolia, a Dashamula source, leaves resembled roots in chemical fingerprints and in anti-inflammatory and analgesic activity. For Shyonaka (Oroxylum indicum), decoctions of both root bark and the stem bark recommended by the Ayurvedic Pharmacopoeia showed significant anti-inflammatory activity. Young stem had principal flavonoids similar to root on mass-spectrometric profiling. For Myrica esculenta, Acacia catechu and Litsea chinensis, small branches resembled stem bark or heartwood in phytochemical screening and HPTLC profiles, but pharmacological equivalence was not tested. Ashwagandha illustrates the limits of the approach. Leaves contain higher concentrations of reactive withanolides than roots, and Indian authorities have restricted ashwagandha leaf in food products and directed manufacturers to use root only. Plant-part substitution is a promising conservation strategy, especially for slow-growing trees in Dashamula and similar groups. It must, however, be validated by chemical similarity in relevant constituents, equivalence in activity for the classical indication, and comparable safety. We propose a stepwise validation framework and recommend that validated substitutions be adopted in pharmacopoeial monographs.