Changing column question..

I just want to make sure that from what I understand I can not change a column type from varchar(20) to text or
anythingelse without dropping the table.  From my reading would the easies thing to do be to create a temp table
exactlyidentical to my current table.. copy all of the information over from one table to another.. (is there an easy
SQLcommand to do this.. I've done from one column in table a to one column in table b.. but never the whole table).
Dropmy old table.. recreate it in the new format.. then copy all of my information back? 

Also,
    Where can you decide/change postgresql's behavior on dealing with trying to put too much information in a column
witha type of varchar(20).. I want it to insert everything up to 20 chars then drop the rest.. currently it dies with
anerror about trying to put in to much information.. 

Travis