Only one word in the first line of text.

[beric]

I have a problem. Now I have hundreds of large files that look like this:

word1 word2 word3 word4 word5
word1 word2 word3 word4 word5
word1 word2 word3 word4 word5
word1 word2 word3 word4 word5
.....

(Some files starts with a blank line...)

The first line of each file is the name of the HTML file. There can be only one word and no spaces. How do I change all the documents that the first line was only the first word of whole text? :

word1
word2 word3 word4 word5
word1 word2 word3 word4 word5
word1 word2 word3 word4 word5
word1 word2 word3 word4 word5