It won't get weaker (or stronger). The air in the headspace can cause oxidation. The wine can change color and get a "sherry" flavor to it- that's the trademark of oxidized wine.
I usually top off with a similar wine, white wine for light wines, red for dark wines, or with water if that's built into the recipe formulation. I often go from a 6 gallon carboy to a 5 gallon while racking, and keep the remainder in a small wine bottle just for topping up.