It's a pretty difficult subject, but i'll take a crack on the transition from Tsarist to Soviet Russia and law here, i might write a followup on Soviet to Republic if i have time (it's an easier question). The question is also partially answered here by me (but i basically expand upon that here) and by /u/Thepurplepantywaist who gives a more general perspective on the continuity of law.

First i'll give some points on Marxist notions of law, as these are important both in the period and kind of divide how scholars see the period. Secondly i'll talk about the revolution and war communism (1917-1921/22) and after that the NEP period (1921/22 -1928). The debates on the characteristics of Soviet law follow largely into the early Stalin period as well, but i will not go over that.

1. Marxist and Bolshevik views of law

Marxist is used in a broad sense here, not only referring to what Marx himself wrote, being not especially preoccupied with it nor educated in law.

Law in a Marxist analysis is typically subservient to the economy. A normal viewpoint is that of a "base-superstructure", where the base of society determines it's superstructure, though there is some reciprocity. Law is a part of the superstructure, while economy is a part of the base. Thus, a Marxist could say that law is an extension of entrenched class positions within the economy system. Law is inherently political. As Engels put it in Socialism: Utopian and Scientific:

The economic structure of society always forms the real basis from which, in the last analysis, is to be explained the whole superstructure of legal and political institutions, as well as of the religious, philosophical, and other conceptions of each historical period

Engels thought that the state machinery would simply be less relevant after a revolution and change of economic system, that a different economic structure would render the old political and legal system irrelevant, it would (eventually) wither away. This must, however, also be seen in conjunction with the idea of a dictatorship of the proletariat, the assumption of political power by the proletariat class.

Lenins views to a degree reflect this base idea to a degree. In his 1917 State and Revolution he makes the same point that all law has a class character, so it was necessary to move away from the old state machinery and towards revolutionary tribunals with more mass participation, but also states that it is necessary for the party to role by an iron hand, which with time would be less necessary. On the other hand, at least once Lenin was in power, he did express a need for a legal system under Soviet control sort of as a necessity to ensure Soviet goals and power, which could be reasoned as the proletariat class assuming political power.

Piotr Stuckha, a contemporary bolshevik jurist, also argued along similar lines for a "soviet law" to govern the transition period. Another jurist, E.B. Pashukanis thought law built on capitalist notions of contract relations, (state/citizen, worker/labourer, wife/husband) and that law was this intrinsically linked to capitalism, thus even soviet law was bourgeois law. The Soviets could use law for their purposes, but the eventual goal was it's complete removal. In the early soviet period in general we see some conflict between these notions of whether the Soviets should strengthen the legal system and their own control or weaken it to precipitate the "withering away".

So Bolshevik thinking was not uniform, but emphasised both an idea of the withering away of law, both as a quite immediate goal or as a long term goal, and the assumption of political power and usage of the legal system to further party and/or proletariat goals

2. The Revolution and war communism

I'm starting here from the bolshevik revolution, not touching on the february revolution. Initially, the ideas of removing the old state machinery as an embodiment of the old class system was dominant. Decree number one on courts of the Council of Peoples Commissar's 24th of November 1917 abolished all civil and criminal Tsarist courts, judges, prosecutors, investigators and attorneys as a profession, abolishing the bar (advokatura), this meant essentially that everyone had the right to be a "lawyer". The only types of courts under the decree were revolutionary tribunals and local courts (peoples courts).

It's a bit unclear how Tsarist laws are treated under this decree, as far as i understand they were not outright abolished and were allowed to continue to be applied in local courts (as these dealt with local and less important matters), and were cancelled when they contradicted central decrees or revolutionary legal consciousness, which gave judges a bit of discretion where they were expected to uphold the revolution.

What practical effect this had, say in the rural countryside where the communists lacked control, is a more difficult questions. I don't have any studies of it on hand, but it's reasonable to assume that a lot of legal customs probably continued as before when the Bolsheviks didn't force through change. A source of mine says that many peasant judges just continued or were elected or elevated by the people to continue. Another source says that crime at least increased substantially in Moscow due to the collapse of the earlier judicial system, which indicates that the changes did happen at least in Moscow.

Furthermore, very early on in the revolution (30th of october), the same body had issued a decree on the official procedure for the publication of new laws, following Tsarist procedures. This illustrates that the Bolsheviks were happy to take over existing political systems where that was better than abolishing them and at least had some intention of using the legal system. Furthermore the new government relied to a large degree on legal specialists from the previous administration in accomplishing this task.

You mention the early constitution, the constitution of 1918, and the creation of that, while it can be seen as a propaganda piece, also to a degree illustrates the idea of at least using the legal and political system for class and/or party purposes. I won't go too much into the constitution.

The Bolsheviks soon changed their mind on some of the early abolishment of courts, and saw it such that the situation called for a system to provide law and order. By decree in February 1918 the Advokatura was re-established, with a membership that included both former Tsarist lawyers (though, not if they had been condemned as reactionaries), but also included others, even many non-lawyers if deemed qualified. Later decrees in November sought to establish a legal profession based on state salary and state control, but the result was a more chaotic overlap of state and private practice, which a decree in 1920 more or less allowed. Further decrees were also given to strengthen the court system, notably in May, July and November 1918.

So the law of especially this early period is a bit of a mess and quite hard to find good sources on, the goals of the bolsheviks seem unclear and disputed in various writings. To me it seems like the Bolsheviks had a quite pragmatic attitude towards law and was mostly concerned with ensuring their political position and goals. So that, along with the general chaos of the Civil War period, explain why the Bolsheviks abolish part of the legal system and lawyers associations while later seeking to again create a functioning legal system.

Continued below