[Reclassify/add dimension to bill line item]

I would like to create a SmartEvent that codes all expenses (i.e. account number greater than 4999) to a specific dimension (i.e. class ID: 20). 

I've entered the API per below, but it is failing. What am I missing?

<update>
<APBILL>
<RECORDNO>{!APBILLITEM.APBILL.RECORDNO!}</RECORDNO>
<APBILLITEMS>
<APBILLITEM>
<CLASSID>20</CLASSID>
</APBILLITEM>
</APBILLITEMS>
</APBILL>
</update>