Only a few weeks prior to the 1956 Hungarian Revolution, the Soviets agreed to allow Poland to enact reforms as part of the Polish October. Soviet and Soviet-appointed officials were removed from the Polish government, restrictions on the Catholic Church were lifted, and the 1957 elections had more candidates than seats. As part of the agreement, Poland promised to remain within the Warsaw Pact and not abandon communism. The Hungarian Revolution was far more radical than the Polish one and actually overthrew the government, but as late as October 30 the Soviets planned to leave the new government in place and even considered withdrawing troops. However, the Soviets decided to intervene the next day after Hungarian politicians began calling for a withdrawal from the Warsaw Pact. Clearly the Soviets were willing to allow some significant changes within their Warsaw Pact allies during the 1950s. How far might they have been willing to let them go, especially if the Hungarian Revolution had been more similar to the Polish October?
Well, considering the fact the Polish elections were not really in any way an actual reform, just a standard communist-organised quasi-democratic event, very typical for the immediate post-WW2 era, I'd say that's as far as they were willing to go. Establishing a popular movement that's entirely controlled by the one pro-Soviet party is a very common theme in the early history of the Eastern bloc, even outside the Warsaw pact, as having a semi-populist agenda was a very important method for keeping in power.