how to read a text box value using jquery

i am using a select control while showing all the availble items from the database,but when i want to add a new record i am converting the select in a text box.After converting the text box i am unable to read the value of it

select declration

*Board ID

converting to text box

function Add() {

$('#cmbBoardID').replaceWith($("", { type: "text" }));

}

reading the value

Var Boardid = $('#cmbBoardID').val();

getting undefined while reading value as above.Any suggestions would be helpful .

Thanxs in advance