How to cast as type of a table column?

I have multiple views with union like this:

select column1, column2 from mytable
union
select cast(extcol as nvarchar2(15)), cast(extcol2 as nvarchar2(10)) from exttable

But later I change the size of column2 to nvarchar(20) and now I need to change cast(extcol2 as nvarchar2(10)) to cast(extcol2 as nvarchar2(20)), in every view or reference that could be exist.

I tried it:

select column1, column2 from mytable
join
select cast(extcol as mytable.column1%TYPE), cast(extcol2 as mytable.column2%TYPE) from exttable

It throw me an error:

SQL Error: ORA-00911: invalid character
00911. 00000 -  "invalid character"
*Cause:    identifiers may not start with any ASCII character other than
           letters and numbers.  $#_ are also allowed after the first
           character.  Identifiers enclosed by doublequotes may contain
           any character other than a doublequote.  Alternative quotes
           (q'#...#') cannot use spaces, tabs, or carriage returns as
           delimiters.  For all other contexts, consult the SQL Language
           Reference Manual.

What can I do?