How do I change the extension of a long filename quickly?

original file: a really really long name with spaces.ext target file: the same name.new-ext command: mv *part of the name that'll give a unique answer.ext $(echo $(ls -l | grep -i *.ext | cut -d ' ' -f 11-)) result: mv: target 'last word of the name.ext': No such file or directory What am I doing wrong? PS: the remote machine only allows sh (not even bash) so tab-completion is out of the picture.