In this case, there is no substitute for reading the manuals.
You will want to read through the installation and upgrade instructions
for OpenVMS Alpha V7.0 as it will contain prerequisites including a list
of OpenVMS versions one can upgrade to V7.0 from, and you will want to
read through the new features and release notes manuals for each of the
intervening versions, and you will want to read through the installation
and upgrade manuals for intervening versions. There have been many
changes to OpenVMS Alpha since V1.5 -- far too many to mention here.