ID's must be unique, therefore there is no point in using #Form2
$("#UserID").change()
however since you have duplicate ID's, things will not work properly.
a MUCH slower solution would be to use
$('#form2 :input[id="UserID"]').change();
I suggest not having duplicate Id's.
edit: Also, you could use the name attribute of the input:
$('#form2').find('input[name="UserID"]'); // this assumes you give the input a name of UserID
-- Kevin