When the question for my answer here was changed to CW, I think my answer should have been made CW as well.
It's not retro-active. I'm not sure it should be. If you provide an answer that was thoughtful, constructive, and helpful to many other people, you should be allowed to accept up-votes and in turn receive reputation. This should all happen regardless what the OP does with his question.
Existing answers aren't changed to CW when the question is. It's up to the OPs to change them - though I think the moderators can go in and change them manually too.
As other noted, the only "wiki-hammer" occurs when a question reaches 30 answers and is not already community wiki. At that point the Q and all As are forced into wiki.
Thus, any poster can opt in to wiki at the time of post, without affecting any other questions or answers.
(it is not possible to opt out of wiki, ever, though.)