Aligater made a good point - what would happen if being put on ABM didn't make a difference to her weight gains? I'd be asking your CHN that if it ever comes up again. I'd be making a point of saying that you could end up ruining a perfectly good breasfeeding relationship for nothing if you make the switch and nothing changes.
DD2 started full time formula at 7 months, and her weight gains stayed the same as they were when she was fully breast fed (and she didnt sleep any better either ;o))