Yeah, pretty surprising move, because Russian government doesn't really change much, even when their performance is going down. Russian government is very protective for it's composition, statesmen rarely go work outside, they just change their positions.
>govt resigned in order to Putin can acquire their power
Not really. Government and even parliament in Russia is nothing, they do what Kremlin want. Government can't show any dissent, and it doesn't even have any serious people who can oppose Putin, they are bunch of oligarchic managers, nothing more. Parliament (both Duma and Federation Council) are machines that support literally everything that is said from "top".
So, resignation isn't directly related to all these constitution changes, especially considering that it would be done not now, but in some distant future. Some sources say that Kremlin didn't like how government worked in last years, but it is strange reason - government does same for much longer time. Other sources say about multi-d-chess and other things. New prime minister is relatively unknown person, who worked as head of federal tax service.
But actually, I don't thing that anything would change.