As the answer by user6726 says, copyright is automatic when an original work is "fixed in a tangible form (such as a computer file). This is now true in all countries that adhere to the Berne Copyright Convention, or the WTO's TRIPS Agreement. That included all but a very few countries in the world.
Under US law, the copyright must be registered before one can start a suit for copyright infringement. Statutory damages are not available under US law unless the work is registered before the infringement occurs, or else within three months of the publication of the work.
If an earlier version of a work is registered, a later version will usually be a derivative work of the earlier version, and so the registration of the earlier version will suffice if the later version is infringed. However, a defendant could argue that the later version is not a derivative work, and so the earlier registration is not valid for the later version.
The greatest possible protection is obtained by registering each separate version as a separate work, But there is a fee for each registration, and there is time and effort expended in preparing the registration submission, and in tracking the registrations of each version. How much money, time and effort to expend on registrations of different versions is a business decision, not a legal issue. Some holders will register every version, some only those versions where significant changes are introduced, and some only one early version.
A new version will not extend the duration of copyright on an earlier version.