Can you switch formula types but stay in the same brand?

Hi ladies... First of all I pump and supplement with formula because my supply isn't high enough to meet the demands of my baby. Right now we use Enfamil infant. I know you're not suppose to mix around with formulas because it may upset their stomach- but I'm noticing he gets a hard, firm stomach after and a lot of gas. He doesn't seem to be in pain but can look pretty uncomfortable. Would it be ok to try the Enfamil Gentlease for his grassiness? Or should I just stick to what he's on so I don't make him even more uncomfortable? Thank you