Meagan Longoria explains the process around table alteration when it’s a temporal table:
System-versioned temporal tables were introduced in SQL Server 2016. They provide information about data stored in the table at any point in time by storing an effective dated version of each row rather than only the data that is correct at the current time
You can alter a temporal table to add or change columns, but you must first turn off system versioning. Let’s look at an example.
The example here relates to a computed column, so a bit more work has to happen due to the way you define computed columns.