How do I modify XML? I know where it is, but cannot figure out how to modify it

I basically created a temp XML document and put the XML from both files (base and page) into 1, so that I could freely move data around as desired.  It got me past that first hurdle.

Ben comments his code very well, but he is far above the level of ColdFusion developer I am.  It would be like a grade school math student attending a class in advanced trigonometry.  It's only a solution if you can comprehend what he's doing at each stage, and even with his comments, I was not able to follow his solution.  It can be as simple as verbage throwing me off; if we're not on the same page, I simply can't follow.

So for example, I have a structure now like this:

(LOCAL.mergedXML):

<root>

     <template>

          <!-- The base template XML is here. -->

     </template>

     <pageTemplate>

          <!-- The merger template XML is here. -->

     </pageTemplate>

</root>

I was successfully able to use arrayAppend() in order to append information from the <pageTemplate> node into the <template> node, but oddly, I am now executing the next command, to prepend data, and when I attempt to do so, ColdFusion is throwing an error.  The code it errors on is:

<cfset arrayPrepend( LOCAL.mergedXML.root.template.xmlChildren[ LOCAL.tplVars.matchedIndex ].xmlChildren, LOCAL.tplVars.prependContent ) />

I have verified through isArray() that both values are arrays, but CF returns a: java.lang.ClassCastException error with an empty message value.

As for why I'm building my own template engine, Pablo Picasso once said "I am always doing that which I cannot do, in order that I may learn it".  I understand coder's desire not to "reinvent the wheel", however standing on the shoulders of those who came before me may get me to a solution faster, but does not expand my skillsets as a developer.