Rebuild xml with xslt : copy all elements and apply function on some attributes

Here is a sample message:

<?xml version="1.0" encoding="UTF-8"?>
<root xmlns="http://ift.tt/r5ut6F">
    <id>xxx</id>
    <atom:entry xmlns:atom="http://ift.tt/r5ut6F">
        <atom:id>atom_id_01</atom:id>
        <atom:content type="application/xml">
            <event xmlns="http://event" xmlns:sample="sample" id="event_id_01">
                <sample:product a="a_01_before" b="b__01before" />
            </event>
        </atom:content>
    </atom:entry>
    <atom:entry xmlns:atom="http://ift.tt/r5ut6F">
        <atom:id>atom_id_02</atom:id>
        <atom:content type="application/xml">
            <event xmlns="http://event" xmlns:sample="sample" id="event_id_02">
                <sample:product a="a_02_before" b="b_02_before" />
            </event>
        </atom:content>
    </atom:entry>
</root>

1. Extract entry node


2. Apply a function (ConvertUtil.change(String value)) on product/@a and prodcut/@b.

The result should be like:

<entries>
    <entry xmlns="http://ift.tt/r5ut6F">
        <content>
            <event xmlns="http://event" id="event_id_01">
                <product a="a_01_after" b="b_01_after"/>
            </event>
        </content>
    </entry>
    <entry xmlns="http://ift.tt/r5ut6F">
        <content>
            <event xmlns="http://event" id="event_id_02">
                <product a="a_02_after" b="b_02_after"/>
            </event>
        </content>
    </entry>
</entries>

I tried in many ways and found out the namespaces were so annoying. Do I need to remove namespace first?