In Oracle you have two approaches. If you have full control over the database server, I suggest you just change the default character set, i.e., the value of the NLS_CHARACTERSET
parameter, and left the rest of the database stuff unchanged. Using regular CHAR/VARCHAR2 columns offers a wider feature set (such as full text indexing).
If don't have full control over the server or you need to keep compatible with old non-multibyte aware applications, you'll have to check the value of the NLS_NCHAR_CHARACTERSET
parameter and alter all your DB stuff to use the NCHAR/NVARCHAR2 data types.
Here's another question that highlights the differences:
Difference between VARCHAR2(10 CHAR) and NVARCHAR2(10)
Last but not least, here you are some useful queries:
SELECT * FROM NLS_DATABASE_PARAMETERS;
SELECT * FROM NLS_INSTANCE_PARAMETERS;
SELECT * FROM NLS_SESSION_PARAMETERS;
Note: I can't help with the migration tool or SQL Server parts, sorry.