I am not aware of any comprehensive list of changes made across the printing history of the 1991 edition. Most, of course, were in the nature of typographical corrections. At some point an acknowledgement was added to Don Bowen "for continuously searching and finding numerous typographical errors in the 1991 revision", so we may suppose there to have been a fair number of these. From Robert's comment on 326 we see that the changes also encompassed the attributions. (Incidentally, there was apparently an intermediate step in the change to 326 -- I have a book that reads "Caleb Jarns Taylor" and then a later one with "Caleb Jarvis Taylor".)
Below are a few examples, but I am in no way suggesting this is a comprehensive list (particularly regarding any changes to song texts), just to give some idea of the type and range of changes that were made.
A couple of typographical errors in music that were later corrected, to add to the examples noted by Robert, are:
225b -- m. 12, bass rhythm corrected (from two quarter notes to dotted quarter, eighth).
480 -- m. 3, tenor rhythm corrected (from two quarter notes to dotted quarter, eighth). (Change made by late 1996/early 1997.)
A couple examples of songs that had repeat dots inserted or corrected/clarified are 274b (opening measure) and 145t (beginning of chorus). I believe there were other such cases.
The few examples of more substantive changes I am aware of are:
85 -- m. 7, bass part, 1-la changed to 7-sol.
166 -- m. 11, bass part, 4-fa's (on F) changed to 1-fa's (on C).
371 -- first two lines of v.2 changed from "Come, Holy Spirit, heav'nly Dove/With all Thy quick'ning pow'rs" to "In vain we tune our formal songs,/In vain we strive to rise". (Change made by mid-2012.)
448t -- choice note (3-fa) in alto on the word "with" removed. (Change made by late 1996/early 1997.)
450 -- chord on the word "rude" (m. 15) changed.
A couple of text changes that are borderline as to whether they are "substantive" or just corrections:
344 -- last word changed from "years" to "year."
466 -- text underlay in alto part changed (in fuging section).
I have also seen one change that actually *introduced* an error(!):
467 -- opening rest changed from whole rest (which would be correct, to fill a complete measure of 3/2) to half rest. (Possibly the intention was to correct the *placement* of the whole rests, which were hanging from the second line of the staff, rather than the middle line.)
Beyond the few cases noted above, I have no information regarding the dates of any changes.
--David W. (Seattle)