adding fields to a table

Hello,

I have a table with a bunch of records already inserted. When I want to add or remove fields from the, what I've done is produce an sql file, add the fields to the create table directive, and add the fields manually to each record to be inserted. Then I do an

\i db_name

which destroys all the tables and records and then recreates them.  I'm wondering if there's a better way. Namely, I alter the table using psql, which I know can be done. What I'm not sure about is whether altering the table will also alter the records, or will the records inserted previous to the alter table be out of sync with the table.

Thanks for your advice.

Mark