Biological process

positive regulation of regulatory T lymphocyte differentiation, positive regulation of regulatory T-cell differentiation, positive regulation of regulatory T-lymphocyte differentiation, positive regulation of suppressor T cell differentiation, positive regulation of suppressor T-cell differentiation, up regulation of regulatory T cell differentiation, up-regulation of regulatory T cell differentiation, upregulation of regulatory T cell differentiation

Any process that activates or increases the frequency, rate or extent of differentiation of regulatory T cells.

positive regulation of regulatory T cell development

Note that immunologists typically use the word 'development' to refer to cells of B or T cell lineages undergoing the process that GO describes as 'cell differentiation'.