Hi Luca,
The calibration data for the 2445B/2465B and others in this family is indeed stored in the NVRAM. If you have a GPIB interface, there is a short procedure to retrieve (and restore if needed) the calibration data here:
Without the GPIB interface, the next best backup procedure is to take a video while scrolling through the stored NVRAM values using the EXER 02 diagnostic routine (described in the service manual). If you need to restore the values, you will need to program the NVRAM out of circuit. The addresses of the calibration data are described in the above link.
I'm not a fan of doing a drop-in FRAM replacement, because electrically it is not compatible with the Dallas power-up and power-down requirements for which the circuit was designed. I won't re-hash all the reasons here again, but you can search for why in this forum.
-mark
