are they the same ? Swap out no problem ?

Yep. They are the same size. :-)

For some reason mine don't fit properly. M&P 40c. I ordered the backstraps direct from S&W and they told me they would fit. I watched a S&W video that talked about the same thing. The back of the new backstrap does have some differences. When I push it down it spreads open. Any ideas?

They should be the same... I would contact Smith and talk to them about it.