A turbine bucket tip clearance control system includes a rotor assembly having a rotor supporting a plurality of axially spaced wheels, each wheel mounting an annular row of buckets, the annular row of buckets on at least one of the plurality of axially-spaced wheels having a radially outer tip shroud provided with at least one seal tooth. A stator assembly includes a radially inwardly facing, axially-stepped surface, formed with radially inner and outer seal surfaces connected by a shoulder. The stator assembly and rotor assembly are moveable axially relative to each other, enabling selective positioning of the at least one seal tooth radially opposite one of the radially inner and outer seal surfaces to thereby selectively alter a clearance gap between the at least one seal tooth and the radially inward facing axially-stepped surface.