I have flown with Sophie pretty frequent since she was 6 weeks old, and have always had to switch planes so I feel like I am a pro at this now. I've lost count but I think she's been on 20 different planes at this point!
I have only been on a handful of flights that they have made me take Sophie out of her ERGObaby during taxi, and it's usually older flight attendants. I have never had to take her out during turbulence, seat belt lights, etc. (I fly with United/Continental) Sometimes the FA's don't even notice.
Also, some planes don't have changing tables in the bathroom. (Like the little cabinet thing that comes down)
Every 757 I have been on and the little commuter CRJ-200's don't. I don't understand why the 757 doesn't since it's a bigger plane :/
I flew with a Bjorn until Sophie was 6 months I guess, it got too painful. The ERGO has a strap you can unbuckle around your waist when you sit down (but the baby is still "secure" for the most part, I mean don't stand up like that) but the Bjorn you have to pretty much completely leave it on. My back used to KILL me

I also like the ERGO because it has a zipper in the front that I stick my boarding passes in.
I don't know how formula works (or how long you can leave it out) but if it were me, I would do it BEFORE you got on the plane, only because I have spilled breastmilk all over me (and almost on the person sitting next to me) trying to transfer milk into a different bottle when I was afraid to nurse haha.
Also the "packet" baby foods are good because you can use the boon spoon attachment or just squirt it directly in their mouth. (Plum Organics, Ella's, Happy Baby, etc --- the boon spoon attachment works on those too)
Give the bottle (or binkie) during take off and landing to help ears from getting stopped up....