How to use sed to change file extensions?

Question

I have to do a sed line (also using pipes in Linux) to change a file extension, so I can do some kind of mv *.1stextension *.2ndextension like mv *.txt *.

Answer 1

Another solution only with sed and sh

printf "%s\n" *.ext1 | 
sed "s/'/'\\\\''/g"';s/\(.*\)'ext1'/mv '\''\1'ext1\'' '\''\1'ext2\''/g' | 
sh

for better performance: only one process created

perl -le '($e,$f)=@ARGV;map{$o=$_;s/$e$/$f/;rename$o,$_}<*.$e>' ext2 ext3