Hello Patty,
I asked my son's pediatrician at his 12 month check up this very question, as I had suspected that the Next Step Enfamil was primarily a marketing tool. He confirmed my suspicions and said that unless my son was significantly underweight or anemic, I could make the switch to whole milk. Of course, your child probably will not take a cold turkey switch, so we began by adding the whole milk to his formula, gradually increasing the amount of milk and decreasing the formula.
My son is now 15 months and is actually exactly on target for where he should be (which is better than he was at 12 months when he was slightly under weight).
All that being said, my sister has an 18 month old and she is still giving him a milk/Next Step mix since her son has had some stomach ailments and is now significantly underweight.
Just ask your pediatrician for advice @ your 12 month. I COMPLETELY trust ours, so had no problem taking his advice to not waste the extra $ on the Next Step and just go over to whole milk. Good luck!