Payment for ecosystem services (PES) have been increasingly applied worldwide to reduce environmental degradation, provide ecosystem services and enhance biodiversity in forests. However, some PES programs in forestry promote monoculture planting, which results in lower biodiversity and fewer ecosystem services compared with mixed-species forests. To address the issue of policy design change while maintaining participation to the program, we investigate participants' willingness to accept a follow-up program featured with mixed-species planting. We use a choice experiment applied to the Grain for Green Program (GFGP) in China. Results suggest that majorities of farmers are willing to stay. Using mixed logit models, we explain farmers' preference heterogeneity for contracts. Results indicate that participants who choose to leave are likely to have smaller cropland area, larger household size, lower total income, and higher non-agricultural income. They tend to have lower institutional trust towards local institutions and perceive insufficient economic benefits and increased land tenure insecurity from their previous experience with GFGP. Willing-to-stay respondents can be divided into two groups. Those who are young, male and perceive increased economic benefits from their past experience, prefer to enter contracts featured with mixed trees with economic value, and to receive moderate or high level of technical support. Other respondents prefer contracts characterized with mixed ecological-economic tree planting, longer contract duration and moderate technical support. Our findings suggest that policy design should shift from merely increasing forest areas to actively promoting more biodiversity, and such policy changes do not lower farmers' participation.