Mixing different baby formulas.
The only time I have mixed formula was after a hard learned lesson about a cold switch from one to the next. If I have to switch formula, which I try not to do, I add less and less of the old formula each day until completely switched to the new. It takes a while but the alternative is awful on my poor Charlie's tummy. If you're switching to or from Soy, I would not use this method. Best to talk to your pediatrician before making any changes to your babies diet.