J.H.
just gradually mix the two-- so for instance if he's on a 6 ounce bottle, do 2 ounces of the new stuff to 4 ounces of the old stuff for several days, and if he doesn't have any issue, then 3 oz new/ 3 oz old, then 4 oz new/2 oz old, then he should be switched over. i would allow a good 3-4 days between each time to be sure he's not getting constipated, upset stomach, etc. i have used nestle in the past (with my first, who is now 4 years old), and i didn't like it b/c it's hard to mix up (the powder anyway)-- you have to be very thorough with it, and it might almost be a good idea to make up a days worth in a larger container and use a whisk or something like that-- i had a hard time getting it mixed properly just shaking it. for that reason also, i would actually make up 2 ounces of it and mix it in to 4 ounces of mixed similac, instead of doing a scoop of it and 2 scoops of similac into 6 ounces of water. of course, if you can do the premixed, that solves a lot of the problem :) i know he's doing good on the similac, and you hate to mess with it, but all formula is fda regulated, so try not to worry. but if you can find a way, like k.p. said, around this-- then i would try that! one of my good friends is using nestle now b/c breastfeeding didn't work out, and she tried enfamil premium, enfamil gentalese, similac, and is now on nestle b/c that's been the best for her little man's tummy. good luck!