We used Alimentum up until a few weeks ago. We did a gradual transition. Like - replaced 1 ounce in each bottle for a few days, then 2, then 3 etc, etc. If it seems to trouble her you can stop and stay at like say half and half for a while.
I noticed though that the Alimentum is SO much thicker, so when you start to mix them and get to where it is more regular similac - it is gonna come out of the nipple quicker. We use number 3 Dr Browns and had to go to number 2 cause the regular similac will run right out of the 3 tipped upside down lol.
HTH,
Good Luck
Zoey