WHEN did the appearance of the quarter rest in music change from its original appearance - as a backward looking number 7 -( ) to its current look ()? I have been searching the Internet with no luck. I know the original quarter rest looked like a backward eighth rest, so the design was changed completely. I just cannot find anywhere WHEN the change took place. It would also be nice to know WHO decided on the change.
Possible duplicate of: Origin of the 'squigly line' used for quarter note rest?
There are several scans of manuscripts (from Corelli to Mahler) linked from that page that show many intermediate forms. It seems to have been a gradual evolution due to handwriting, rather than a dictatorial all-at-once change. In addition, each composer had their own handwriting, of course.
As a whole, they show that the backwards-7 quarter rest gained a 'check mark' on the bottom, in addition to the upstroke and crossbar. The upstroke got slanted in various directions, and the crossbar became curved, which can create a form closely resembling the modern form.
What, unfortunately, hasn't quite been demonstrated on that page is the final step from the modified squiggle to the modern printed form. For that answer, I think you'll have to look at the history of music printing, and see what symbols were being used by the publishing houses, printing presses, and "music typewriters". This page might provide a good jumping off point.
I have "researched" quite a bit and collected some interesting examples, see below. As can be seen, we have a huge geographical impact, France (I have numerous additional examples) seemed especially fond of the backward-7 notation, keeping it up to the middle of 19th century, while the Bach autograph from ca. 1723-1730, the Walsh print of 1795 and the Liszt-Diabelli example I added to the Sqiggly-symbol question point to earlier migration to a new symbol decades or even a century earlier.
Jancourt print 1849:
Bach autograph, ca. 1723-1730:
Walsh print, unknown composer, 1795: