In the context of the European crisis in conventional milk production, conventional farms potentially dissimilar to organic farming models are converting to organic. This raises the issue of farm vulnerability during and after this conversion, i.e. the farm's ability to respond to the effects of technical, climatic and economic risks. Our objective was to show whether and how dairy farm vulnerability can decrease during and after conversion to organic farming. We surveyed a sample of 12 dairy farms in Brittany, France, from 2008 (their last year conventional) to 2013. Our method considered farm vulnerability a function of the initial level of and trend in farm self-sufficient milk productivity, economic efficiency, net profitability per worker and independence from European common agricultural policy (CAP) subsidies. We related these vulnerability variables to explanatory variables that illustrated farm exposure to climatic and economic variability and farming practices. The results show that nearly all farms improved their economic efficiency (11/12), about two thirds improved their self-sufficient milk productivity (8/12) and profitability per worker (7/12), and half improved their independence from CAP subsidies (6/12). Farms had diverse vulnerability patterns, with trade-offs between the initial situation and the trend followed during the conversion, and among vulnerability variables. We identified two main adaptation strategies: (i) pasture-based farms that were similar to organic farming models when conventional and that did not change much during the conversion, and (ii) farms based on maize and feed concentrates when conventional that drastically changed. The latter farms had the greatest decrease in vulnerability and improved their self-sufficient milk productivity, profitability per worker, economic efficiency and independence from CAP subsidies. Overall, variability in climatic and economic conditions had less influence on vulnerability than farming practices. Here, we showed for the first time that changing farming practices by converting to organic farming can be a powerful mechanism for reducing farm vulnerability.