ALTER TYPE Not Available So How To Alter a Type?
Simplest would be to create a new type with the updated definition, then you can change the affected objects one at a time (or all at once) to reference the new name. You can’t pull the tablecloth.

I wrote an article about this here:


How to Alter User Defined Table Type in SQL Server


The fundamental process is:


create a new type
determine which objects reference the old type
change those objects manually to point at the new type (or at least those objects that need to use the new type)
optionally, drop the old type


The tricky part is #2, people will hunt and peck manually, or try to parse OBJECT_DEFINITION or sys.sql_modules, but the following is a bit more reliable:

SELECT s.name, o.name, def = OBJECT_DEFINITION(d.referencing_id) 
  FROM sys.sql_expression_dependencies AS d
  INNER JOIN sys.objects AS o
     ON d.referencing_id = o.[object_id]
  INNER JOIN sys.schemas AS s
     ON o.[schema_id] = s.[schema_id]
  WHERE d.referenced_database_name IS NULL
    AND d.referenced_schema_name = N'dbo'
    AND d.referenced_entity_name = N'MyType';