SQL Server 2017 Enterprise - how to fast update entire columns
maybe some sort of metadata trick is possible?


Adding a new column with a default can be a metadata-only operation.
Dropping a column can be a metadata-only operation.
Renaming a column can be a metadata-only operation.

I'm not certian that all of these always are metadata-only for every type of table, but  try something like:
  use AdventureWorksDW2017
  go
  drop table if exists DimCustomer2 
  go
  select c.*
  into DimCustomer2
  from DimCustomer c
  cross join (select top 100 * from sys.messages) d
  go
  set statistics io on
  go
  begin transaction
  alter table DimCustomer2 add LastName2 nvarchar(200) not null default 'HIDDEN'
  alter table DimCustomer2 drop column LastName
  exec sp_rename 'Dbo.DimCustomer2.LastName2', 'LastName', 'COLUMN'
  commit
  go
  set statistics io off
  go
  select top 100 * 
  from DimCustomer2