The difference between VARCHAR2 and NVARCHAR2 is the number of bytes used to store a character. Since, as was mentioned, the physical size in bytes of the area in which these 2 field types get stored in in the same, capacity is halved for NVARCHAR2.
Although you can swap out tables to get around this,
you should also be able to play games with just the original table. Just make sure that you have a readily-accessible backup!
Basically, what you'd do is alter the table to add a column with the new field type defined, then apply a SQL update to copy the original column's value into the new column, applying any code-page mods or the like as you do. Then alter the table to remove the original column and rename the new column to the original column's name. Reorg the table as needed.
If the original column does, in fact contain items whose length is greater than 2000 characters, then your new column would need to be a CLOB, not NVARCHAR2. Because no matter how you shuttle the data around, you can't put 2 tons of
fertilizer in a 1-ton truck.