TracProjectProperty

import hudson.model.*
import hudson.plugins.trac.*

for(item in Hudson.instance.items) {
   println(item.name)
   def newTracProp = new TracProjectProperty("https://trac.colo.lair/trac/")
   def tracProp = item.getProperty(TracProjectProperty.class)
   if(tracProp != null && tracProp instanceof TracProjectProperty) {
       println("old-> "+tracProp.tracWebsite)
   }
   tracProp = newTracProp
   println("new-> "+tracProp.tracWebsite)
   item.setProperty(tracProp)
}