Sql server 2016 Change only server and without all databases
You'll need to run setup with the REBUILDDATABASE option to change the server collation without changing the collation of existing user databases. This will create fresh system databases so you'll need to re-create server-level objects, re-attach user databases, and remap logins afterwards.

Note the tempdb collation will also be changed and can result in collation conflict errors after the operation depending on the existing user database collations and how queries are coded (e.g. creates temp tables without an explict DATABASE_DEFAULT column collation specified).

A system collation change is not a trivial matter and needs to be thoroughly tested.