Yes, any time you reduce your number of nursing sessions you can expect that your supply will eventually go down. But if you keep your daytime sessions, your baby should be able to maintain your daytime supply at the point where he wants it to be, regardless of what happens (or doesn't happen) overnight.
Night weaning does not mean losing your magic tool! You can use nursing to soothe hurts and tantrums and crankiness and even to get your child into a sleepy mood at bedtime. You're just giving it up in the middle of the night.