If you have a system running a pre-9.0x
version of HP-UX, and you are willing to destroy all system customization
in order to install 10.20, then cold-installing
10.20 may be the right answer.
Remember that HP-UX 10.20 uses pathnames and file locations
that are different from HP-UX 9.0x. When
upgrading, back up your current system in such a way as to separate
"structural" (system) directories from "data" (user and application)
directories. This will allow you to recover data directories onto
10.20 without compromising the 10.20 structure.
You should not recover any pre-10.0 "structural"
files onto the 10.20 system. Recover them only if the 10.20 install
fails for some reason and you need to restore the pre-10.0 version
of HP-UX.