Can I change the recording and Q&A settings of a webinar after it's been scheduled?

It depends on the setting and where you change it. 

For Q&A, if you change the default setting in the general settings area, it will apply to new scheduled sessions. 
But you can always edit an existing scheduled session directly and check the Q&A box.

Same for recording if you are changing the default for ex "auto record". You have to update it in the scheduling form  itself. If you change whether the user is allowed to start local or cloud recordings, those settings take effect when the session starts, regardless of when it was scheduled before. 

So the distinction is if the setting is also exposed in the form, or just in the general settings area. 

Updating a scheduling form does not impact already registered people (unless you change registration settings specifically) and their links will work just fine

After the session has started, settings changed in the general area will not take effect, and you are blocked from editing the scheduling settings of a session that's already in progress.