B diffuses in crystalline Si by reacting with a Si self-interstitial (I) with a frequency g and so forming a fast migrating BI complex that can migrate for an average length lambda. We experimentally demonstrate that both g and lambda strongly depend on the free hole concentration p. At low p, g has a constant trend and lambda increases with p, while at high p, g has a superlinear trend and lambda decreases with p. This demonstrates that BI forms in the two regimes by interaction with neutral and double positive I, respectively, and its charge state has to change by interaction with free holes before diffusing.