I was wondering whether later versions of Gregg improved on something in previous versions or if they fixed flaws in previous versions.
The basic trend seemed to be towards reducing brief forms but were there any significant improvements (beyond brief reduction, assuming you think of that as an improvement)?
The main purpose of series after the 1916 New and Revised Edition (which was the most abbreviated form of Gregg ever published) was to simplify the system, not so much to improve or fix flaws.
In Anniversary, some outlines were modified, some word beginnings and endings were eliminated, and other principles were eliminated.
Starting with Simplified, the main purpose of the revisions was to have the student writing shorthand earlier in the course, though not necessarily faster. This was done by reducing the number of principles considerably and relying on writing more words in full form, as gaps in the knowledge of theory would decrease speed. So instead of giving more practice on the existing theory, the authors decided to decrease the number of principles for each of the series, under the belief that the less theory the student has to learn, the better.
Carlos is right about this, though Centennial seems to have been developed to rescind some of the over-simplification of Series 90. But even with the general trend toward overall simplification, there were some improvements made. For instance, Simplified is more consistent in its spelling than Anniversary (a point Louis Leslie seems to have considered a strength of the system). Also, some writers had been clamoring for a short way to write rd to parallel the ld of Anni and Preanni. Simplified supplied it.
Diamond Jubilee attached -ed and -er consistently. In previous series, many of these forms were detached, and often needlessly. Though DJ is generally slower than Simplified and earlier series, the attached endings do tend to speed things up, both because of theoretical consistency and because no lift of the pen is needed.
Even the much maligned Series 90 made a couple of haphazard improvements over Diamond Jubilee. I can think of three:
* First, "doctor" was written again as d-r when DJ had it written in full because d-r stood for "during". (Okay, so S90 took away the short form for "during". But if you're going to keep one and lose the other, surely "doctor" should be d-r.)
* Second, an attached g for "graph" was introduced. Simplified did away with "egraph" and "ograph" and kept "gram". But "graph" seems to be more common than "gram", so the loss of that ending was a bad thing. S90's new ending corrected this. Unfortunately, it was lost again in Centennial.
* Third, "without" and "throughout" were spelled consistently again. Up through Simplified, they were e-th-t and th-r-u-t, respectively. In DJ "without" was written e-th-a-u, while "throughout" stayed th-r-u-t. S90 respelled "throughout" as th-r-a-u. Since "without" is much more common that "throughout", it makes no sense to spell "without" nearly phonemically but to maintain an irregular form for "throughout". Thus S90's solution was an improvement.
Pre-Anniversary supplies a way of writing the Welsh "ll" sound, which does not occur in English. Do you need that? Sometimes cutting unnecessary features is an improvement in itself.
Ditto the gutteral aspirant kh. This occurs in Russian, German, Welsh, and Scottish, among others, but not in English (unless you think Scottish is a dialect of English; but that could start a flame war). Perhaps these sounds were given written forms in case anyone pronounced Loch Lomond or Llewelyn in the native style.
Does the march towards simplification mean that if you learn an earlier versions, reading later versions is easy? And that if you learn series 90 reading the other versions is difficult because you would know so few briefs?
What version to version transitions are hardest and which the easiest?
In a nutshell, I answer yes to your first two questions. And it is not only due to briefs, but also phrases, word beginnings and endings, and above all, the abbreviating principle!
As to your last question, let's classify Gregg series into three big groups:
Group 1: Series up to Anniversary.
Group 2: Simplified
Group 3: DJS, S90, Centennial
The easiest transitions would be within members of Group 1 or within members of Group 3, followed by transitions across Group 3 to Simplified, then Simplified to anything in Group 1, and lastly, the hardest would be from Group 3 to Group 1. This is a generalization, and in any generalization, your mileage may vary. For example, you could argue that there are easier series within Group 1 (for example, the first edition), but even using the first edition as an example, it contained a reporting style that further abbreviated the forms and made it somewhat comparable to the other members of the group.