Sundry additional game details (date, round, board, teams, event date, player titles, ratings, and so on) were added in mnay cases, and all annotations included in the November 19, 2017 version were retained (except that clock times and other live board artifacts were removed).
It would take too much space here to describe all the changes in detail. Instead a complete accounting of all differences between the update and the old version icu-20171119-163001.pgn (from November 19, 2017; missing only 24 games from this year's Bunratty Festival that were added on February 26) is contained in the file icu-update-with-comparisons-2018120-v0.pgn. This is a valid pgn file and should be playable using any pgn reader. It can also be useful to read it with text editors such as Notepad or Notepad++: the formatting is cleaner and it's possible to search for and count patterns across the entire file.
Here is one example that illustrates the format of the comparison file:
[Event "Oxford"] [Site "Oxford ENG"] [Date "1970.??.??"] [Round "?"] [White "Corden, Martyn"] [Black "Moles, John"] [Result "0-1"] [PlyCount "20"] [ECO "C15"] [Opening "French"] [Variation "Winawer (Nimzovich) variation"] [ICUid "15940"] [HashCode "1587bd42"] { Changes: "Site" changed: "?" -> "Oxford ENG" "ECO" changed: "C01" -> "C15" >> "PlyCount" (= "20") added >> "Opening" (= "French") added >> "Variation" (= "Winawer (Nimzovich) variation") added >> "HashCode" (= "1587bd42") added movetext change: was: (1. e4 e6 2. d4 d5 3. Nc3 Bb4 4. exd5 exd5 5. Qf3 Qe7+ 6. Nge2 Nc6 7. Be3 Nf6 8. O-O-O Bxc3 9. bxc3 Bg4 10. Qf4 g5 11. Qxg5 Bxe2 12. Bxe2 Qa3+ 13. Kb1 Ne4 0-1 ) --- end of changes } 1. e4 e6 2. d4 d5 3. Nc3 Bb4 4. exd5 exd5 5. Qf3 Qe7+ 6. Ne2 Nc6 7. Be3 Nf6 8. O-O-O Bxc3 9. bxc3 Bg4 10. Qf4 g5 { 11. Qxg5 Bxe2 12. Bxe2 Qa3+ 13. Kb1 Ne4 } 0-1
The URL of this game is http://www.icu.ie/games/15940. This is captured in the pgn file via the new tag pair [ICUid "15940"]. For any game in which the movetext (the part starting with move 1 above) changed in any way other than line breaks and extra spaces, the old movetext is included in the comment preamble. For the game above, the change is that the game actually ended after 10... g5, and the extra moves should have been shown as a possible continuation.
(The update contains many pgn tags (ICUid, ICUType, and others) that will not be retained if the pgn file is converted to .cbv format. If you convert to .cbv format and then convert back to pgn, you will find that about half the tags are deleted, and sometimes that new tags are added. However the games themselves will be playable.)
The changes take up over 250,000 lines, but the vast majority are minor: repetitive (e.g., event names, player names) or automatically generated (e.g., ECO code, opening, plycount, player name format).
Acknowledgements: The update also incorporates an earlier significant overhaul prepared by Tim Harding that added 1,288 new games, in addition to many changes of the types listed above. David McAlister helped resolve several particularly awkward discrepancies. And Jonathan O'Connor made several very useful changes to the database storing the games archive on the ICU web site.
Finally, it's virtually certain that there are many remaining errors lurking in the update. If you notice any, we'd be interested in hearing from you: please send corrections, and any new games, to [cf. ICU page].