Is it possible to change recorded keynote timing after recording manually?
You can't do this in Keynote.  Keynote recording only allows for you to record the whole narration at once.  You could work around this by recording your audio in GarageBand, and then importing your GarageBand audio into Keynote.  GarageBand allows for much more finely-grained control of your recordings, including re-recording a section.  

Another option is to use PowerPoint:Mac, which does allow you to re-record the audio for one slide.