Stefan,

No copyright license ever restricts the rights of the original author; it only describes which of those rights he chooses to grant to others and under what conditions. If he chooses to grant certain rights to others, he cannot rescind such a grant once given, but that certainly says nothing about what he himself can or cannot do.

Anything you write is yours. Any part you choose to give to others is usable by them under the terms of you give them. If you at some later time want to use it for a proprietary product, you are free to do so. If you had previously granted some rights to use the text to others (say by publishing it under the GFDL), those people still retain those rights to that version, but you still retain ALL rights to that version and any subsequent ones.

FOr example, everything I write I give to the public domain. I simply release all my rights to everyone. My work, as amended by other Wikipedians and published there, is only licensed under the more restrictive terms of GFDL, but that doesn't mean I can't also publish my own work on my own site fully in the public domain (as I intend to do). I can also sell it to a publisher if I wanted, though I can't "take back" the versions that I've already given away. To negotiate in good faith, I would have to disclose to a publisher that there existed free versions of what I was selling them, and perhaps they would ask me to rework the text so it could be copyrighted more restrictively. I wouldn't care to do that, but I certainly could. 0