Once you have been in the hobby a while you will realize that having other finished wines around can help with a lot of things that happen
ok let me explain that, if you need to top off a carboy it kind of matters what you use. If you use water then you are watering down the wine, Using a wine will allow the wine to stay at alcohol level, it can affect the taste a bit but I do not think a Concord will be changed by so little of an addition.
I have even used a bit of grape juice to maintain taste when topping up. Remember to keep notes to refer to, Having that knowledge base to refer back on is invaluable.