1. DS was horribly gassy. We started with Enfamil, went to Gentlease which helped for a few months, then ultimately ended up with Gerber Good Start, by which point he'd been down to mostly BM and one formula bottle (as opposed to 25/75 formula in the beginning). Some of it was probably just him getting bigger, but I think Gerber did help.
2. We mixed the 2 formulas for a few days before going with just the new one.