Don't toss it. I make 5 gallon kits in 6 gallon carboys all the time. The key is to transfer to a 5 gallon once you get too around 1.020 to 1.010. That way you protect your investment.
I would recommend getting a 5 gallon before the wine gets that far, even a 5 gallon better bottle would be better than loosing your wine.
Now, this is all if you're bulk aging in the carboy, if you're going to do like me and once its finished you stabilize it, clear it and bottle it and let it age in the bottle.