I have been asked to update the software version on a Juniper NS-IDP-250 to the latest version available (5.1R3).
Actually there is the version 4.1.112010 running on that device. On http://www.juniper.net/support/products/idp/#doc , the manual says that in order to install the 5.1R3, the device requires me to install the 5.0R2 first. The manual about how to install the 5.0R2, also says that the 4.1R4 has to be installed first... and so on.
At the end, I've understood that there is this path to follow like this: