Switching Formula Mistake

I am a new mom and think I already made a mistake regarding formula switching.
I breast-fed for 6 weeks then decided to supplement with Enfamil Lipil with Iron, DHA, etc.  We used it for about 4 days but it seemed to give the baby constipation so we switched to the Nestle Good Start with Iron.  We've been giving it to her for one day - - and a 40 minute melt-down later I'm beginning to realize my mistke of not gradually introducing the new formula. My question is should I just push through and continue to give her the new formula or should I back-track and give her some Enfamil and some Nestle Good Start?  I also plan on breast feeding (1/2 breast milk and 1/2 formula).  

Thanks

[newbie2b]

I switched my daughters milk three times due to different reasons. First similac, then Enfamil, now good start soy.  Each time I switched I talked with the Dr. and was told that it was fine to do it "cold Turkey" as she put it.  It worked for us. No trouble.  Just be sure to monitor her for the first week to rule out any allergies.  

[mum2beagain]

Every baby is different, some tolerate it well, others don't. Especially for a breastfed baby the introduction of formula can be a bit of a shock to the digestive system and often can cause constipation and gas. I have just started to give my 9 1/2 month old formula, I started with lower iron, now I am mixing !/2 lower iron and half with iron, when these cans are done I will just have him on the iron fortified type.
It might be worth mixing the 2 formulas, first with a greater quantity of the Enfamil and gradually working it up until it is all goodstart.