This has been portrayed as a simple attempt for Putin to hold on to power but there is actually much more behind these constitutional changes, and it is not as extreme a government shake-up as the media were alluding to. The resignation of the prime minister and his cabinet is just a technicality and many of the ministers that were forced to resign will in fact be repositioned into new government roles, with many expected to retain there positions directly. The Russian economy has stagnated for 6 years and Putin has failed to reform the economy as he promised when he took power, but this stagnation has worsened under Prime Minister Dmitry Medvedev, and Putin is now putting a technocrat, Mikhail Mishustin, in his place.
These constitutional reforms will however give parliament more powers, including the future power to elect the prime minister, while the President's powers would be curbed and handed to the Russian Parliament. Putin is getting ready to stand down and increasing oversight and curbing future potential to stay in power as long as he has, this should strengthen the Russian political process more than it undermines it. In addition, Russian voters seem to back this move, Putin remains popular in Russia but this government were losing popularity and were increasingly viewed as ineffective by the majority of Russians. Ultimately these constitutional provisions will be decided by Russian citizens in a referendum, this is not the first time something like this has occurred in the country, it's just unexpected; Putin keeps his cards close to his chest.