Conversion between NVARCHAR to VARCHAR

I've got an Oracle DB with ALL the character columns defined as NVARCHAR or NCHAR or NCLOB, using charset UTF-16.

Now I want to migrate to a new DB that has charset UTF-8. Since it can store unicode characters, I'm wandering if I will be able to import data converting column types.

The reason I'm doubtful is I know that I cannot convert a NVARCHAR2 column in a VARCHAR2 if not empty.

What is the best option to perform the import. Will datapump complain if I import the schema, modify the column types and after that I'll import the data?

Thank