Abstract: Using forest survey data collected from 2015 to 2020 in a 12-ha permanent monitoring plot in Guanshan, Jiangxi Province, China, this study examined how contrasting forest conversion pathways reshape community structure and stand development. A space-for-time substitution approach was applied to compare two representative transitions: conversion from Cunninghamia lanceolata (Lamb.) Hook forest to broadleaf forest and conversion from broadleaf forest to Phyllostachys edulis (Carrière) J. Houz (moso bamboo) forest. Species richness, stem density, diameter-class structure, height-class structure, mortality, recruitment, and stand volume were analyzed to clarify the trajectories and potential drivers of community change. During conversion from C. lanceolata forest to broadleaf forest, species richness increased, whereas stem density showed a V-shaped pattern. In contrast, moso bamboo expansion was associated with continuous loss of species richness and a gradual increase in stand density. Diameter-class and height-class distributions in all forest types generally exhibited reverse J-shaped patterns; however, the two conversion processes showed contrasting structural trajectories. Broadleaf forest conversion promoted a substantial increase in tall trees over five years, whereas moso bamboo invasion simplified diameter and height structures and gradually replaced large-diameter trees with bamboo culms. In both conversion pathways, tree mortality exceeded recruitment. Stand volume increased during broadleaf forest development but declined under moso bamboo expansion. Overall, contrasting forest conversion processes exhibited distinct community dynamics.