A word or phrase for what happens after a convergence
It seems like the word singularity is exactly what you are looking for, as it describes multiple things coming together to become one. It has also in recent years been used to refer to the hypothetical future of computers becoming conscious, and so it may have connotations you do not wish to have in your writing. Perhaps, in your world, society has reached a singularity which caused it to cross the event horizon, beyond which nothing can ever be the same? Literally speaking, an event horizon is the point where a black hole's gravity becomes too strong to escape, but you could use it to get across the point that things coming together in a singularity changed things in a way that is irreversible.
One of the best ways to describe such a convergence would be the phrase Paradigm Shift.
Paradigm Shift: a fundamental change in approach or underlying assumptions.
For e.g., Einstein's application of Reimannian geometry to the problem of gravitational perturbations brought about an absolutely world-changing paradigm shift in physics. Cosmologists would never again revert to Newton's relatively simplistic concepts of space, time & gravity.
The phrase was coined by the scientist Thomas Kuhn in his renowned book The Structure of Scientific Revolutions which was itself a paradigm-busting study of how new scientific advances actually occurred in history.
for when several "trends" converge - after which, everything changes and you cannot go backwards.
Essentially I am trying to describe where several separate trends/concepts are progressing/advancing, to the point where they converge, and a totally new concept is formed - that isn't merely the sum of the parts.
The newly formed discipline of Chaos Theory was engendered in precisely this manner. Several scientists in varied disciplines and geographic regions discovered phenomena that could not be described by existing scientific lore. They all had to develop new tools to explore & describe these phenomena. And surprisingly, all of these phenomena (as well as the newly discovered tools) were related. Not indirectly or by analogy, but literally, directly related.
In his book, Chaos: Making A New Science, author James Gleick used Kuhn's concept of paradigm shifts to describe the emergence of the discipline.
Have you considered the term a watershed moment/event? A watershed moment is a point in time that marks an important, often historical change.
Example:
The development of nuclear weapons constituted a watershed moment in the history of humanity. With nuclear weapons, now it has become possible to obliterate an entire nation off the earth's surface in split seconds.
Another term similar in meaning to a watershed moment would probably be a sea change. The meanings of the two expression are very similar.
Example:
The past 30 years have witnessed a sea change in the development of personal computing. We have gone from gigantic mainframes of the 60s and 70s to small, compact desktop PCs rivaling in the processing power supercomputers of the 80s.
PS: Another similar expression that just came to my mind is crossing the Rubicon. Look up its origin and history.