PROSPECT-I, the first phase of the Precision Reactor Oscillation and SPECTrum Experiment, demonstrated the observation of reactor antineutrinos in an aboveground detector with good energy resolution and well-controlled backgrounds at a baseline of $sim$7m from the High Flux Isotope Reactor (HFIR) reactor at Oak Ridge National Laboratory. The PROSPECT collaboration is now preparing an upgraded detector, PROSPECT-II, to probe yet unexplored parameter space for sterile neutrinos and contribute to a full resolution of the Reactor Antineutrino Anomaly, a long-standing puzzle in neutrino physics. PROSPECT-II leverages the existing detector knowledge and introduces only major modifications to the inner volume subsystem to improve long-term stability while maintaining the performance to achieve the physics goals of the experiment. This change in the inner volume subsystem removes the internal calibration access tubes and thus requires an alternative calibration scheme. The new calibration approach for PROSPECT-II can be validated by the PROSPECT-I dataset and simulation using a creative selection of active detector segments in the analysis. This talk will cover the detector improvements and provide details of a new external calibration scheme.