In a broad sense, what is happening is that Baltimore's downtown is expanding but also changing in character. Harbor East is and has always a decidedly mixed-use area, blending residential, retail, restaurants and offices. The traditional downtown, meanwhile, is moving in the same direction. The process is slow, owing to the multitude of different property owners and the age and diversity of buildings there. But the transformation of obsolete office buildings into apartments and condominiums has sparked a population boom downtown, and along with it have come more restaurants, entertainment and retail. Mayor Stephanie Rawlings-Blake has sought to foster that transition through legislation that went into effect last week providing property tax credits for some conversion of office buildings to residences.