uf_field api wants "field_name" as required parameter for exiting fields.

I can't test this out on 4.5 because the new api explorer doesn't have Create as an option for UFField. See http://screencast.com/t/REPidQJzh0I

However, in the older versions, I would think that it should be possible to do something of the following

civicrm_api3('uf_field', 'create', array( 'id' => 1, 'label' => 'My First Name'));

This would rename the first name field in the "Name and Address" profile to be "My First Name".

The outcome of the above code is instead and exception that reads: "Params must be a field_name-carrying array and a positive integer."