I assume you mean replace Tuesday Joy with Vigor. But anyway, whatever the case, I'm afraid you can't do that. This is from the rules on page one of the thread..........

The "stable stars" are 3 horses that we pick now and they stay with us right through to the end of the comp, injury, poor form whatever.

So, I'm afraid you're stuck with Tuesday Joy even though she's not going to earn you any points. That's just the way it goes. You can still pick Vigor from the weekly lists though. Don't forget to post your tips for that.