You should have no problem taking train assuming that you arrive around 8AM at Changi.
You did not say where are you going to stay, but for example the first train from Raffles Place towards the airport is at 6AM
https://www.sgtrains.com/guide-traintiming.html
And the train ride
is about 40 min Even if you are coming from Jurong East, you should be at the airport in about 70min. Just make sure that you are on the first train.