How to declare XPath namespaces in xmlstarlet?

I am new to xmlstarlet so hoping this answer is a simple one.

I am writing a script to modify Inkscape SVG files from the command line. I chose the tool xmlstarlet.

After testing the command syntax on test files, I am having trouble on the real SVG files. I think the use of namespaces is throwing me off.

Example file:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!-- Created with Inkscape (http://www.inkscape.org/) -->    
<svg
   xmlns:osb="http://www.openswatchbook.org/uri/2009/osb"
   xmlns:dc="http://purl.org/dc/elements/1.1/"
   xmlns:cc="http://creativecommons.org/ns#"
   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
   xmlns:svg="http://www.w3.org/2000/svg"
   xmlns="http://www.w3.org/2000/svg"
   xmlns:xlink="http://www.w3.org/1999/xlink"
   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
   width="603"
   height="1000"
   viewBox="0 0 159.54375 264.58334"
   version="1.1"
   id="svg8"
   inkscape:version="0.92.1 r"
   sodipodi:docname="test.svg"
   inkscape:export-filename="/home/user/dev/inkscape/test/1.png"
   inkscape:export-xdpi="96"
   inkscape:export-ydpi="96">
  <defs
     id="defs2">
    <linearGradient
       inkscape:collect="always"
       id="linearGradient6204">
      <stop
         style="stop-color:#8f1a22;stop-opacity:1;"
         offset="0"
         id="stop6200" />
      <stop
         style="stop-color:#8f1a22;stop-opacity:0;"
         offset="1"
         id="stop6202" />
    </linearGradient>
  </defs>
</svg>

I want to change Gradient6204 to Gradient9999.

I wrote this command, which does not work (just returns the original file).

xmlstarlet ed -u "/svg/defs/linearGradient[@id='linearGradient6204']/@id" -v 'linearGradient9999' text.txt

I also tried again, adding the namespaces with -N but no luck. I found that if I delete the line:

xmlns="http://www.w3.org/2000/svg"

from the file then the command I wrote above works.

What is the proper syntax for updating the SVG file above in the way I described?