Metastable high-entropy alloys (HEAs) represent a significant advancement in material science, addressing the longstanding challenge of balancing strength and ductility in alloys. The study of metastable HEAs offers a valuable framework for exploring the influence of γ-f.c.c.→ε-h.c.p. phase transformations on mechanical performance, particularly concerning volumetric changes during deformation. Structural changes, such as variations in the c/a ratio, provide critical insights into the interplay between phase stability and mechanical behavior in HEAs. We investigated the microstructure, mechanisms underlying phase transformations, and mechanical behavior of metastable Fe₄₀Mn₂₀Co₂₀Cr₁₅Si₅ and Fe₄₀Mn₂₀Co₂₀Cr₁₅Si₅ + 0.25wt.% B₄C HEAs fabricated via laser powder bed fusion (LBPF). Special attention was given to comparing how the presence of B₄C affects phase transformations, including variations in the c/a ratio, volumetric changes, and overall phase stability during deformation. The HEAs without B₄C demonstrated an increase in the c/a ratio, primarily due to a significant expansion of the c-axis, while introducing B₄C altered this response, leading to a decrease in the c/a ratio. This reduction was driven by a contraction of the c-axis rather than an expansion. Furthermore, the a-axis remained unchanged for both alloys. The contrasting behaviors highlight how adding B₄C fundamentally alters the stress-induced dimensional changes during the γ-f.c.c.→ε-h.c.p. phase transformation.