You could install a second machine for testing, but as you say, it would
be a large project.  It would also be possible to install a second
LISTSERV instance on the current machine, but it may not be a much smaller
project in practice.  You'd have to create a new home directory for the
test instance, compile new binaries with the correct spool path, run
LISTSERV under a different user id, alter /etc/aliases to route mail for
the new LISTSERV user id to the correct copy of lsv_amin, set up a new
virtual host in apache, change the test LISTSERV's TCP/GUI port to
something other than the default, and probably make other changes of which
I'm not thinking off the top of my head.  It would be possible, but kind
of a pain.  Personally, I would just make a copy of your current
~/listserv directory, run the standard 'make update' during off-hours, do
some testing, and roll back to the copy if you don't like the result.  It
would almost certainly be less headache...

--
Liam