Jesus Christ was Himself baptized in the Jordan river to provide an example for each of us. (Reflections of Christ)
Hi friend. It’s obviously important not to lie during your baptismal interview. It would be very ironic to commit a serious sin at the very moment when you’re getting ready to abandon your sins.
That having been said, it seems unlikely to me that you would be excommunicated. It’s important to talk with your Bishop, though, if you have made this mistake. He can help you apply Christ’s atonement in your life. I hope this answer helps.