I've been searching at forum about how to change a textfield readOnly property.

Well, if you want to change it you can't do it trough a Ext method, so you have to access trought DOM and this is the way.

Ext.get('yourTextbox').dom.readOnly = true;

Ext.get('yourTextbox').dom.readOnly = false;
It's not a big thing, but I hope that will be useful to you.