Can I keep default 2.3.1 from 2.0
I saved a configuration I had from a 2.0 installation and later restored it onto a new 2.3.1 device.
Obviously, there have been many changes and new configuration options. I am wondering if it is safe to assume that if my 2.0 config was working fine that the restore and new default options are also fine?
When you restore an old config, the system reboots to implement it. As it loads the old config during boot it discovers that the version of the config is old, so it applies all the config upgrade steps to bring its format up to match the current format. And it works.
That applies to even ancient configs from V1 series.
The main thing is that you restore a complete valid old config - do not restore parts of an old config into an existing new config.
What Phil said.