Anyone switch to medium flow nipples yet from slow flow?

sam&tm

We've been using them with all his bottles today & he definitely finishes the bottle quicker & isn't falling asleep while eating (I think he falls asleep because it takes him about 30 to 45 minutes to finish w/ slow flow) but he's spitting up after his bottles. He's never spit up before. Do you think it's will stop once he transitions?or him finishing a bottle in 15 minutes is too fast & we should go back to using the slow flow?