rename s/\.txt/.doc/ *.txt
Or if you want to use a more sophisticated regex to avoid documents like this-has-a-.txt-that-is-not-the-extension.txt being changed into this-has-a-.doc-that-is-not-the-extension.txt
then that's fine as well. But that's an okay regex for 95% of the cases.
Oh, also, this isn't a "standard Unix(TM) command, but it does come with most Perl installations that I know of and Perl is on most Unix machines that I know of.