Getting tables from one DB to another

We actually have an application with its database working and we are
developing a new version of such an application based on the same database
but with minor changes in a few tables. When finished, we want to get the
data in the actual DB to the new one but we see a liitle problem. Each table
has a serial field which works as a primary key and we would like to keep
these fields with their original values in the new tables. I suppose I
cannot use pg_restore from a backup because of the difference among some old
tables and some new ones; maybe one way could be to write an application
that reads each record from the old table and inserts it to the new one but
in this case the serial field could be different from the original. How can
I achieve this goal?

Respectfully,
Jorge Maldonado