Drill bit with multiple cutting structures

A bit for drilling a wellbore, comprising:

• a shank having a coupling formed at an upper end thereof;
• a body removably attached to a lower end of the shank and having a blade receptacle formed therethrough;
• a blade fixed to the body;
• a plurality of superhard cutters mounted along a leading edge of the fixed blade;
• a blade disposed in the blade receptacle and longitudinally movable relative to the body between an extended position and a retracted position;
• a plurality of superhard cutters mounted along a leading edge of the movable blade;
• and
• a spring disposed between the shank and the movable blade and biasing the movable blade toward the extended position,
• wherein:
• the movable blade is repeatedly movable between the positions, and
• the bit is configured such that:
• the cutters of the movable blade engage a bottom of the wellbore and the cutters of the fixed blade are disengaged from the bottom of the wellbore while the movable blade is in the extended position, and
• the cutters of both the fixed and movable blades engage the bottom of the wellbore while the movable blade is in the retracted position.