Geosynthetic reinforced soil integral bridges (GRS-IBs) combine a jointless structural system with reinforced backfill mechanically connected to the abutment. The K57+125 highway overpass in Quanzhou, China, is a GRS-IB, modified from a jointed bridge. This study investigates its design modification, stability verification, and economic performance. After conversion from the original jointed bridge to the GRS integral system, the main-girder prestressing arrangement was redesigned, reducing the number of tendon groups from five to four. Internal stability verification showed that the maximum reinforcement tensile force was 13.84 kN and the minimum pullout safety factor was 96.05, while the reduced ultimate bearing capacity and the reduced dead-load bearing capacity corresponding to 1% vertical strain both exceeded the applied vertical pressure of 97.59 kPa. The sliding and overturning safety factors were 2.18 and 5.92, respectively. Compared with the original bridge scheme, concrete and prestressing-strand consumption decreased by approximately 36% and 12.76%, respectively, while reinforcing-steel consumption increased by approximately 18%. The estimated initial construction cost was nearly 10% lower. These results demonstrate the structural feasibility and economic potential of the adopted GRS integral bridge scheme.