Renaming the "base" property can happen via file-system (YAML file) or using the backend user interface of the "sites" module.
Changing the "base" property results in different URLs for all nested pages (or slugs in general). This should result in having redirects being created automatically (or based on a user confirmation dialog - which is not possible if YAML file has been modified directly).
Updated by Frank Naegler almost 3 years ago
I would opt for a read-only field with a hint why this field is read only (after first save) as a first step.
Adding an info why you should not change the base and what steps must be done in case you really want change the base.
This list could later be used to build a kind of upgrade wizard to change the base in the backend.