A few cool hints for renaming from the Mini HOWTO: Batch renaming.
To change all .htm in .html just type
for file in *.htm ; do mv $file `echo $file | sed 's/\(.*\.\)htm/\1html/'` ; done
and in general
for file in [filenamepattern] ; do mv $file `echo $file | sed 's/[search pattern]/[replace pattern]/'` ; done