You're exactly right. Just open up each file, copy the source to a new WordPad file, and save it. It's not a big deal; they do not say you must destroy the software created, but the FILES created. And this may very well be because the files are in a proprietary format, so they don't want you using them for other things, or somesuch. But again, there is nothing here to worry about: they cannot claim your files themselves for their own, and they cannot prevent you from copying the source to other files.
The only slightly troubling thing is the word "immediately," but that is IMO sufficiently vague -- obviously, it cannot expect you to do it immediately, since you may not even have access to your computer at the moment the license is terminated -- that it wouldn't forbid the time it took to copy the data out to other files. And presumably you are backing up your data; if you are that concerned about it, look into a backup script (written in ActivePerl or something that has no such terminology in its license :-) that will automatically copy your data to files of a different type, so if the time comes, you'll already have the backed up, legal, files.