CJ, depending on where you were in the fermentation, it could reduce the ABV by as much as 10% so that if your ABV was 12% it would be down to 10.8%. Also, for your future batches, you do not necessarily need to top off the carboy when going from primary to secondary. There is still active fermentation so that there will be a blanket of CO2 on top of the wine and, of course, you will have an airlock in place. Topping off, or as I do, "sizing down" to a smaller container(s), is important when fermentation is complete and you are stablizing and clearing your wine.