According to some Korean hardware sites, Phenom II x3 710 and 720BE with 0904xxxx stepping can be transformed to Phenom II X4. (0849xxxx steppings seemed to not work, and other steppings are not imported in Korea, so no idea for other steppings.)
It needs following conditions.
1. working with 790gx&sb750-based boards.(Asrock and Biostar boards are confirmed. Asus seemed to not work)