Question

I examined how ExtJs renders form’s fields in dom. To change fieldLabel after field is rendered I find proper dom element and change its innerHTML;

/**
* Modifies field's label afrer field is rendered.
*
* @param {object} field
*    Some object that directly or indirecty extends Ext.form.Field
* @param {String} label
*    New value for field's label.
*/
function setLabel(field,label){
    var el = field.el.dom.parentNode.parentNode;
    if( el.children[0].tagName.toLowerCase() === 'label' ) {
        el.children[0].innerHTML =label;
    }else if( el.parentNode.children[0].tagName.toLowerCase() === 'label' ){
    el.parentNode.children[0].innerHTML =label;
    }
    return setLabel; //just for fun
}

//EXAMPLE:
var win = new Ext.Window({
    height : 200,
    width : 300,
    layout : 'form',
    labelAlign : 'right',
    items : [{
        xtype : 'textfield',
        fieldLabel : 'name',
        ref : 'f',
        html : 'asdf'
    },{
        xtype : 'datefield',
        fieldLabel : 'date',
        ref : 'd'
    },{
        xtype : 'combo',
        fieldLabel : 'sex',
        ref : 'c',
        store : [[1,"male"],[2,"female"]]
    },{
        xtype : 'radio',
        fieldLabel : 'radio',
        ref : 'r'
    },{
        xtype : 'checkbox',
        fieldLabel : "checkbox",
        ref : 'ch'
    }]
}).show()

setTimeout(function(){
   setLabel(win.f,'Last Name')(win.d,'Birth Date')(win.c,'Your Sex')(win.r,'jus radio')(win.ch,'just checkbox');
},3000);

Answer 1

Accessing the dom to dynamically change fieldLabels was pre 3.0.1

After 3.0.1 I believe this works

field.labelEl.update('New label');

You can only do this once the field has been rendered.