To get a copy, use the dup (or clone for < rails 3.1+) method:
#rails >= 3.1
new_record = old_record.dup
# rails < 3.1
new_record = old_record.clone
Then you can change whichever fields you want.
ActiveRecord overrides the built-in Object#clone to give you a new (not saved to the DB) record with an unassigned ID.
Note that it does not copy associations, so you’ll have to do this manually if you need to.