There is currently no known way to reverse faction with TS Centaurs. Since there is constant content development, I'd expact that to change sometime, probably not soon.
Some things you can do in the meantime:
- Grind out levels until they /con grey. I'm a 28 CONJ and 80% are grey to me.
- Ask a high level guy hanging around if you can join to gain his /con. They cents will turn grey , but you run the risk of your "protector" going link dead or disbanding, leaving you at the feet of a snarling centaur with a rough agate in your hand.
Instead of a standard "everything agro's, go kill" it routine, the centaurs were set up with an extra twist. You could remain non-agro until you killed one - then the free ride is over. However, I am able to go just about anywhere in TS zig-zaging around.
Bottom line is, your con is changed and won't go back. The up side is that there are MANY quests to kill centaurs. You can even pick up 2-3 different quests and get update pings for all of them by killing just one.
The Watcher of the Plains talky centaur will still give you quests. He just doesn't know how many of his bretheren I've slain. :)