bash is cool
We know this functionality is there, but often forget.
IF we have a directory full of folders with plone_*
for i in plone_*; do mv $i iavtheme_${i#plone_}; done
will replace plone_ with iavtheme_ for each directory name
See these for other pattern toys (thanks Juan for reminding me of this useful stuff I never seem to use):
${variable%pattern}
Trim the shortest match from the end
${variable##pattern}
Trim the longest match from the beginning
${variable%%pattern}
Trim the shortest match from the end
${variable#pattern}
Trim the shortest match from the beginning