how did the soviet union change after stalins death?

It changed a great deal, there was a general liberalising of the arts and more of an acceptance of dissent (within limits), there were massive changes in the way that agriculture was organised, the way that the Party was organised and, importantly, the way that the Party dealt with it's opponents - it would no longer execute them. This curtailed the power of the NKVD, as did the closing of the Gulag system of camps: the last shut in 1960.

As for rules to prevent another Stalin. The Party did not put in place any rules. But what it did was to remove Khrushchev from office through an extra-ordinary meeting of the Presidium of the Party. He "voluntarily" retired from power. The Party had developed a mechanism to remove a future dictator.

Initially the urgent first step was the arrest and liquidation of Beria before he could take over.

The NKVD/MVD/SLON etc continued until the Kruschov reforms and the end of the cult.

After that there was a rapid emptying of a large part of the GULAG system and the leadership became subject much more to the collective elite of the party.

From there it was decay and corruption until the East German party lost their nerve and perestroika came in.

-It went from Worse to Bad... Which STILL wasn't much of an Improvement. :(