I don't think there's a quick fix, but it can be done. If you already have the hub in a word processor like Microsoft Word, it's easy. Just open the file and select everything that you want to change to lower case. While your text is selected, go to the Word menu at the top of the screen, go to "Format" and select "change case". Select the "sentence case" option so that the first letter of each sentence remains in caps. Once this is done, you can go back to your hub's edit mode and paste blocks of text back into your Hub over the top of the old text. Don't forget to proofread your article to capitalize proper nouns as you will convert them to lower case in this process.
If you don't have your hub saved in a word processing document, you'll have to cut and paste back into a word processor to make the changes I've outlined above. Otherwise, you'll have to retype in the correct case.
I'd be interested to know if anyone has an easier way to do this?