Hi M.,
I'd recommend looking into Baby Pottying to ease her struggles. Some babies have very strong instincts not to poop on themselves, and offering them a potty opportunity makes their day! It has been known to ease a lot of baby belly troubles, as they 'get it all out' rather than over a series of diapers.
How to Start EC? 7 Quick and Helpful Tips:
http://tinyurl.com/Starting-Baby-Pottying
"Our baby girl would cry when she needed to poop. Now she looks at me with a silly grin on her face because I'll put her on the potty and say "poop. poop." and make farting noises with an animated face. Both of us really enjoy ECing. Thank you C. for your supportive and inspiring website.
Chelly, Washington, USA"
Check it out here: http://www.parttimediaperfree.com/
I can't comment on the formula switching only to say perhaps that is an aspect of it.
- C.