Can you switch a 5 month old from a soy based formula back to a milk based formula?

My 5 month old was switched to soy formula from milk formula due to gas and cramps when she was 6 weeks old. She has done great with the soy in that way(no more cramps or gas), but she has had chronic constipation issues from the soy. I try to talk to the doctor and she just tells me to give the baby more juice. That works sometimes, but it is not a fix to the overall problem.I hate to see her strain and suffer to push out a few small pebbles of poop! I was wondering if now that she is older, maybe milk formula would be ok again.Anyone have any experience with this or ideas? Thanks!