Can I restore a pfSense configuration to a different version
Configurations are specific to a given pfSense version. The version of FreeBSD used is not relevant. The platform being used (full installation or NanoBSD) is also not relevant, the configuration is the same on all platforms and architectures using the same pfSense version.
Generally speaking, a complete/whole older configuration version can always be restored to a newer release of pfSense. The configuration will be upgraded as needed provided that the entire configuration has been restored.
A newer configuration cannot be restored to an older release that had a different configuration version. Certain releases of pfSense had the same configuration version, and restoring between those is possible, but still not recommended. See Versions of pfSense and FreeBSD to see which configuration versions were used on specific pfSense releases.
A configuration section or partial configuration cannot be restored between different configuration versions. It may work by pure luck, but often there are configuration format differences that require changes to be made to the older configuration. These changes are automatic if a complete configuration is restored. If a partial restore is required, perform a full upgrade in a test VM or lab and then copy the needed section out of the resulting config.xml post-upgrade.