You can still report all the posts you consider to be spam and moderators will probably delete most of them if you created that topic.
Sometimes it takes a long time to get handled. Also, there is no guarantee that my report will be handled. I believe this is one of the main reasons you want to create a self-moderated thread.
It wouldn't be fair to change topics from normal to self-moderated later and I think scammers could abuse that function.
Just lock the old topic and create ate new one that is self-moderated... it's not that hard.
Both arguments got some good points. I agree with this statement as well. As others already discussed, if old posts don't get affected, I don't see any harm in the implementation of this system. Imagine you created a topic and it has more than 20 replies already, then you realize you wanted to make it self-moderated.