Polarization manipulation of electromagnetic waves is of paramount importance in a wide spectrum of applications, including but not limited to radar detection, satellite communications, and advanced wireless systems. To date, the development of reconfigurable polarization conversion devices that simultaneously achieve high conversion efficiency, broad operating bandwidth, and high-level functional integration has become a critical unmet demand in this field. In this work, we design and experimentally demonstrate a broadband, multifunctional, reconfigurable polarization conversion metasurface integrated with PIN diodes. By independently tuning the ON/OFF states of the two loaded PIN diodes, we enable the proposed metasurface to achieve fully dynamic switching between multiple polarization manipulation functionalities, specifically linear-to-circular (LTC) polarization conversion, circular-to-linear (CTL) polarization conversion, linear/circular-to-cross (LC/CC) polarization conversion, and stable polarization retention.In detail, LTC and CTL polarization conversion are realized across the 9.2-12.1 GHz frequency band, while LC and CC polarization conversion are achieved within 7.8-10.6 GHz and 7.9-9.6 GHz, respectively. Meanwhile, stable polarization retention is obtained over the 8.4-11.3 GHz band. Notably, a polarization conversion ratio (PCR) higher than 0.9 is achieved for all the aforementioned operating modes. The underlying physical mechanism of the proposed polarization manipulation is further elucidated through eigenmode analysis and surface current distribution characterization. The results confirm that the multimode coupling between electric and magnetic resonances is the core contributor to the broadband and high-efficiency polarization conversion performance. Full-wave electromagnetic simulations show excellent consistency with the experimental measurements of the fabricated prototype, and this agreement rigorously verifies the feasibility and effectiveness of the proposed metasurface design. With its broad bandwidth and versatile reconfigurable capabilities, this design holds great application prospects in emerging fields such as polarization imaging, anti-jamming wireless communications, and radar stealth technology.