Will default value specified in rails migration overwrite column values in the future?

I have a migration which adds a foreign key to the users table.

[timestamp]_add_group_id_to_users.rb

Inside this migration file, I'm specifying a default value for it:

def self.up
  add_column :users, :group_id, :integer, :default => 1
end

But what happens after a while when let's say a user has now group_id: 4 and another feature will be pushed to production?

Will all the migration files be run again? Will group_ids of users be overwritten?

Should I specify something like "overwrite it if group_id is nil but don't do it if it's not and if it's different from 1" ?