Hi, B.. In general, you should not switch formulas at all unless your baby has a problem with the formula you are using.
Is there a specific health reason why you are using Enfamil Premium Lipil? Something like milk allergy, etc.? If so, then definitely do not switch formulas, especially without talking to your pediatrician.
Some babies can be very fussy. If they get used to a certain formula, they will not eat a new one unless they are starving. They all have a somewhat different taste, and the baby will know the difference. It's not worth the tantrum at feeding time unless there is a very good reason to switch. Trust me. I've been there.
There is another issue, too. Quite often, switching formulas can cause colic, quite serious colic. The different formulas have different levels of proteins, fats,and vitamins. If baby's stomach and intestines are used to one kind of formula and you suddenly give him something radically different, it can take time for his system to adjust.
If you really have your heart set on switching formulas, you could try mixing the old one with the new one, half and half, and seeing how baby reacts to this. If he's OK, Then put more of the new formula in, and if this works, try him on the new one alone.
You can also try going to the Enfamil website and seeing if they can give you coupons, too. The formula you are already using sounds really good, but I realize that saving money on everything is important.
I found that getting the formula in powder and then mixing it with sterile water saved me some money (and some muscle strain from carrying cans of formula). It's kind of a pain because the powder takes time to mix up, but it's one way to cut down on expenses.
Peace,
Syl