NLS_CHARACTERSET - Changing columntype to NCHAR

We have the problem of converting a database from WEISO8859P15 to ALL32UTF8.

On the import side CHAR (VARCHAR) fields turn out to be too small due to multibyte character conversion taking place.

I have some fundamental questions:

1. Is the NLS_CHARACTERSET global to a complete instance (SID) or is it possible to have schema or session wise character sets?

2. Can one change e.g. VARCHAR, VARCHAR2 or CHAR fields to NVARCHAR, NVARCHAR2 or NCHAR columntypes dynamically

    in a schema? And if so, could anyone provide a script for such?

Thanks,

Christoph