The consensus is yes you can upgrade from Solaris 2.3 to 2.5.1. Thanks

for the warnings about having enough space in your partitions,

deinstalling disksuite, NIS+ of 2.3 incompatibility with NIS+ of 2.5.1,

etc. Ofcourse a good backup before starting was recommended!!

Many thanks to all who responded, including:

    Steve Turgeon

    David S Foster

    Normand Ranger

    Dean Humphrey

    Viet Hoang

    Kevin Sheehan

    Marina Daniels

    Glenn Satchell