First of all, I don't think there is any canon way to continue your campaign with your characters as they are. But my knowledge of the Mage rules is very limited.
This feeling comes from White Wolf's general attitude with canon being that the GM is free to ignore any part they don't like and from the fact that the New World of Darkness is not a sequel or a future version of the old one. As Jadasc says in his answer.
Those two points gives White Wolf's writer no reason to allow players to do this.
However, here's a generic answer to your question :
Send the players back in time
A bit of googling about Mage and time travel does not bring any hard evidence that it is impossible. This forum makes me think that true time travel is not possible with the player's power, so they would probably need external help, but you do not need to break canon.
And throwing your characters into the past can make for very interesting stories if you let them use their knowledge of the future to their advantage or indirectly interact with their past self.
I have no experience to actually GMing those kind of game. But you should probably be ready to limit the kind of things they can do with the usual paradox.