I will go over the stages in the social history of the USSR in minimal detail, in order to prevent misunderstandings.
The New Economic Policy (1922-1928) consisted of petty private-production combined with state-control of large industries (who operated on "a profit basis"). There was a dictatorship of the proletariat, which retained political control. This is why I am hesitant to call it a "capitalist" society.
The First Five Year Plan was implemented from 1928 to 1933. It was followed by the publishing of Stalin's "Year of the Great Turn" on November 7, 1929, and was a significant shift in policy towards the "law of balanced development" instead of the law of value. Heavy industry, which was unprofitable, was developed in lieu of light industry. This is not something that can be explained by proponents of the "state capitalism" thesis. It also included the collectivization of agriculture, and democratic centralized planning of all of industry.
The mid-1950s to the early 60s signaled the beginning of the Kosygin reforms, which removed centralized planning. Instead, individual enterprises would direct production themselves, respecting a handful of so-called "key indices". Of these "indices", the most prominent was "profit". At this stage, Soviet "economic planning" was more like the "economic planning" in India - that is, to introduce incentives and economic levers in a fundamentally anarchic capitalist economy.
There is no such thing as a socialist mode of production, what we have known since Althusser and Mao is that within any social formation are several different modes of production – in the socialist social formation, there is the capitalist mode of production and the communist mode of production, which coexist. When Soviet commodity production expanded to include not only agricultural produce (as was necessary before the 30s collectivization), but also consumer goods and industrial goods, this is the point where the capitalist mode of production became predominant.
A dictatorship of capital cannot be reformed into a dictatorship of the proletariat, and if socialism were a separate mode of production, a DOtP could not be "reformed" into a DotC. But historical experience has shown this to be untrue. Socialism is not a separate mode of production, and it is entirely possible for revisionism to transform a socialist nation into a capitalist one. It is merely a question of which way the scales tip, in the fragility of class warfare.