Not to address the practical question (ask your Posek!), but people should be aware that soy based formulas are significantly less healthy than milk based ones, and new studies are showing there may be significant health issues involved with consuming large amounts of soy products. Obviously if a baby can’t tolerate the milk based formula there may be no choice, but if it’s due to the CY issue, make sure to speak to your Dr. and get all the information to present when asking your Shailah.
Also, many of the soy based formulas still contain milk products (I believe Similac’s does. . .)