Cast

I made I mistake choosing the data types in my database I try to run a
query that look like this
Duration data type is varchar

SELECT name, SUM(duration) AS total
FROM table
GROUP BY name
HAVING name != 'John'

I get the following error:
ERROR: function sum(character varying) does not exist
HINT: No function matches the given name and argument types. You may
need to add explicit type casts.

1. How can I made a cast on the query from varchar to int?

2. If I change the datatype on the database from varchar to int, do I
loose all the data on that field?

Thanks in advanced