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.
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