All I know the logic where
- new articles will be published to Discourse
- already published to Discourse should be updated when edited
- old articles should not publish to Discourse when edited
sounds a bit confusing.
Without no better understanding about coding than one hello worls test using Virtual Basic, I could guess it can be done comparing status of article against Discourse status, but such actions has tendency to slowing down admin side. Or that would be the situation at some time in pass 
Thinking about settings that would be very confusing setup.