J-PARC Linac accelerates the high-intensity beam of 50 mA using an RF system of 324 MHz and 972 MHz. In order to accelerate and transport the high-intensity beam to facilities stably, the current value, centroid, and distribution of the beam must be measured to realize optimum operating conditions. This paper reports on the transformations and improvements of the linac beam diagnostics since 2015. As an example, carbon nanotubes (CNTs) were employed in the WSM at the upstream of the linac. There has never been an unintentional WSM failure after the CNT replacement. Other reports on the status of BSM operations will also be presented. The diagnosis of beam anomalies experienced during beam tuning will also be reported.