PL/SQL :: Convert CLOB With WE8ISO8859P15 Charset To UTF-8 Charset

I have a CLOB containing a XML code and all XML tags where created using the DB charset: WE8ISO8859P15.

For example:

{code}

<?xml version="1.0" encoding="ISO-8859-1"?>
<certidao xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<funcionario>

[Code]...

I need to maintain this charset value.

But now I need to convert all my CLOB data into UTF-8 charset for external requirements and stored this new CLOB in a table column.

I can't change the charset of the session. I've tried to use a function

{code}
FUNCTION convert_xml (pcl_xml IN CLOB)
RETURN CLOB

[Code]....

This function apparently works, but when I try to validate the XML content (after I replace the encoding="ISO-8859-1" to encoding="UTF-8") the validation software tells me that I have some characters that should not be present in a file using UTF-8 encoding.