How did the change happen, in only about 10 years AFAIK, that all sources I encounter always say form the possessive case of a word by appending “’s”?

Longer than 10 years; I’ve certainly seen forms like that, but they were routinely used in the 19th century. From the following, it wasn’t just one authority that made a single decision; it’s been incremental and exception-ridden, although (from footnotes) the big authorities like Fowler and Hart were influential in advocating against 19th century forms like Brahms’ and James’:

Apostrophe – Wikipedia

Many respected authorities recommend that practically all singular nouns, including those ending with a sibilant sound, have possessive forms with an extra s after the apostrophe so that the spelling reflects the underlying pronunciation. Examples include Oxford University Press, the Modern Language Association, the BBC and The Economist. Such authorities demand possessive singulars like these: Senator Jones’s umbrella; Tony Adams’s friend. Rules that modify or extend the standard principle have included the following:

  • If the singular possessive is difficult or awkward to pronounce with an added sibilant, do not add an extra s; these exceptions are supported by The Guardian, Yahoo! Style Guide, and The American Heritage Book of English Usage. Such sources permit possessive singulars like these: Socrates’ later suggestion; or Achilles’ heel if that is how the pronunciation is intended.
  • Classical, biblical, and similar names ending in a sibilant, especially if they are polysyllabic, do not take an added s in the possessive; among sources giving exceptions of this kind are The Times and The Elements of Style, which make general stipulations, and Vanderbilt University, which mentions only Moses and Jesus. As a particular case, Jesus’ is very commonly written instead of Jesus’s – even by people who would otherwise add ’s in, for example, James’s or Chris’s. Jesus’ is referred to as “an accepted liturgical archaism” in Hart’s Rules.

Although less common, some contemporary writers still follow the older practice of omitting the extra s in all cases ending with a sibilant, but usually not when written -x or -xe. Some contemporary authorities such as the Associated Press Stylebook recommend or allow the practice of omitting the extra “s” in all words ending with an “s”, but not in words ending with other sibilants (“z” and “x”). The 15th edition of The Chicago Manual of Style [2003] recommended the traditional practice, which included providing for several exceptions to accommodate spoken usage such as the omission of the extra s after a polysyllabic word ending in a sibilant, but the 16th edition [2010] no longer recommends omitting the extra “s”.

From Is Jesus’ or Jesus’s the Possessive Jesus?, Jesus’s has been advocated in non-liturgical use at least as far back as the original Hart’s Rules in 1904.

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