By the end of 1917 one autocracy had been replaced by another

Really, this is not an accurate statement.
It is more easy to argue that by around 1930, Stalin had been able to revert back to an autocratic system but that was only through the ground work that had been put in by Lenin.
To first argue that Lenin was an autocrat you have to look later in his "reign". His ban of factions in 1921 is probably the strongest argument that he was an autocrat, as well as his dissolving of the constituent assembly the first time it met in 1918. The ban meant that all had to accept his decisions and could be used to get rid of opposition as the term "faction" was very loosely used. The dissolving meant that this largely democratically elected body was powerless and there was no ground for the public to gain their voice. Lenin's use of terror - most notably setting up the Cheka in 1918 who would go onto murder thousands of people (with 6,000 being killed in October 1920 alone) - is also quite characteristic of an autocrat.
But in truth, in 1917, the autocracy of the Bolshevik regime hadn't exactly emerged. The policy making became much more open and wasn't just the "god-given" right of the Tsar. Lenin did have a sovnarkom which helped him make his policies and was at least a limit on the autocratic power. Though there was no real democracy, it would not be correct to call Lenin an autocrat - Stalin, on the other hand, is a completely different story.