It is important to point out that there is not one — nor has there ever been — Helvetica. In fact, there are so many different Helveticas, it would probably be impossible to list them all. But especially concerning major revisions, what unites them all is the fact that they have always been children of their medium.
Variations started as early has 1959, when the Stempel foundry in Frankfurt began making versions of the typeface for Linotype printing machines. In fact, it was only this — completely redrawn and specially fitted — version of the typeface that initially carried the name Helvetica, in an effort to make the typeface more easily internationally marketable.
When typesetting moved from metal to photosetting, the Typeface again was redrawn and respaced, to fit the new 18-unit spacing grid imposed by the early technology.
When Helvetica moved into the digital medium with the first PostScript printers and software, it again changed. You can probably even tell this from the fact that every modern Macintosh Computer has at least two versions of the typeface installed: The Photosetting sourced Helvetica, and Neue Helvetica (not Helvetica Neue), a redesign of the redesign.
Not really Helvetica...
Helvetica is certainly a very outstanding design: but it is not quite as unique as especially many fans in the United States may have you believe. Not only is it obviously related to it's ancestral AG, but Helvetica also has a prolific Sister, Adrian Frutiger's Univers, which shares much of the same ideas of Helvetica, but resolved them in a different way.
(You can see the similar horizontal terminals in both typefaces. Note the looser spacing in Univers and the different Lettershapes)
And a Love Child:
Other typefaces followed suit, and in the 1970, Haas typefoundry, the original creators of Helvetica, attempted to even release a typeface that caputured and combined the spirit of all these different faces, in something called Neue Haas Unica. Ill-fated due to problonged legal action, it was not until 2015 that a proper release could be made, arguably long fallen out of style and superceded by the onslaught of more modern, humanist sans serif typefaces like Frutiger, FF Meta or Myriad.
Not really Helvetica...Part Two
Many of the Helveticas from the (post) photosetting time are interestingly not Helvetica at all. Because of the unique situation in US copyright law, in which only the physical or digital “font” (the actual physical or digital manifestation, or data) is copyright protected, but not the design, many foundries in the pre-digital era made they own version of Helvetica, that would work specifically with their equipment.
The Unknown Twin:
Another interesting “Sibling”—more like a incestuous child of Helvetica/Akzidenz Grotesk—is Berthold's AG Book. While Berthold claims AG Book's design was based on the original Akzidenz Grotesk, there is a lot that speaks for the fact that AG Book was actually “reverse-engineered” from Berthold's Helvetica Variant (known digitally as Helvetica BQ).
If one compares the lettershapes, the weight and the spacing, it becomes obvious that AG Book is really a Berthold Helvetica with differently angled Terminals in some (not all) letters. The rest of the font, down to the shapes, sizes, and even number of glyphs included matches exactly.
Does this deminish the typeface? I would argue that AG Book is not a bad typeface at all, in fact, by infusing Helvetica with some of the things that made Akzidenz Grotesk so useful, Berthold came up with a very legible and beautiful typeface.