Seconding a recommendation for
https://newarkairportexpress.reachlocal.net/Newark/Faq - I would expect that you should make it with that early bus, as I don't think it will take long.
I believe Uber X to EWR is a bit cheaper than it used to be as well? From my office in Flatiron it estimates $68 right now, I'd assume a little less that early in the morning.
YMMV, but I don't think DL will let you change to a LGA flight unless there is something like a schedule change.