From my experience moving from an E220 to an E420 (which have
the same boards as the U60 and U80), you only have to empty
/etc/path_to_inst (remove everything except for the comment
lines at the top, make a backup first), change the drives and
do a boot -r.

/etc/path_to_inst contains a mapping between the Solaris device
names (e.g. hme0 or glm or ...) and the OBP paths (which will
be different between the machines).  By emptying this file
Solaris will rebuild it upon next boot so it will be correct
for the other machine.

--Frank-Christian Kruegel