This paper presents a case study of the Agricultural Defense System (SIDAGRO) at the Minas Gerais Agricultural Institute (IMA), which underwent a process of modernization involving interface redesign and incremental improvements within an agile development cycle. The objective was to evaluate whether usability and accessibility issues from the legacy system were resolved in the new version and to identify persistent or newly introduced problems. Two rounds of heuristic evaluation, based on Nielsen’s heuristics, were carried out: one on the legacy system and the other on the redesigned system, both after the implementation of key features. The analysis revealed a 43% reduction in the total number of usability problems, with a significant decrease in serious and critical problems. The main legacy issues, such as limited browser compatibility, outdated layout, and lack of interface consistency, were addressed. However, some challenges remained or appeared in new forms, especially regarding system status visibility and recognition of functions. The findings highlight the importance of integrating heuristic evaluations throughout the development process, rather than treating them as a final check, and suggest that an earlier usability analysis of the legacy system could have further reduced inconsistencies in the new platform.