Switching baby's formula..?

My son will be 3 months old in two days and he has been on Gerber Gentle formula for about two months. I know they say not to constantly change your babies formula and I try not to. And this may sound like a silly question. But my husband took my car to work because his was acting up and he'll be gone for a while and we are almost completely out of my sons formula. We do have a back up can of formula that's also Gentle but not the same brand. Friday I plan on going and getting more of his regular formula, but if I have to give him some of the different formula for a day or two and then switch back, will that be a big deal? Will it have a harmful effect on him? It's not like I'm constantly changing his formula but this is kind of an emergency situation. I'm a first time mom and pretty much worry about everything, so any advice from experienced mothers would be extremely appreciated.