Hello Metternich,
We have created a document that describes all the changes in detail here:
https://theministryofgentlemanlywarfare.files.wordpress.com/2021/02/key-changes-between-ihmn-and-ihmn2-v1.pdfTo summarise:
It is eight years since IHMN1 was published. It was in the Osprey Wargames format of 25,000 words over 64 pages.
Since then we have published four other books, all using the IHMN core rules engine, and with each one we made improvements based upon many hours of play-testing, hundreds of battle reports from players, and thousands of posts both here and on our dedicated Facebook groups.
We have put all this experience and knowledge into producing a 196 page, 80,000+ word book, that gives players all they need to run pretty much any sort of VSF & Steampunk games they would like.
I hope that this helps