The China-made telescope, LAMOST, consists of 16 Spectrographs to detect stellar spectra via 4000 optical fibers. In each spectroscope, many movable parts work in phase. Those parts are real-time controlled and managed by field controllers based on FPGA. The master control board of controllers currently being used is constructed by Altera's Cyclone II Development Kit. However, now Altera no longer produce such Kits. As the needs for maintenance and improvement, a backup control board is developed, so that once any field controller is broken, another can changed in time to ensure the control system not being interrupted. Using the newer Altera FPGA chip 3C40 as master control chip can minimize the change in the original design frame of the control structure so as to reduce the workload of software and hardware migration. This paper describes the design process of the Spectrographs backup field controller based on Cyclone 3C40 and gives the problems and solutions encountered during migration for controller hardware and software. The improved field controller not only retains the original controller functions, but also can serve for more motors and sensors due to the increase of input and output pins. Besides, no commodity supply limits, which saves expenses. The FPGA-field controller can also be used in other telescopes, astronomical instruments and industrial control systems as well.