When it comes to upgrading to new version of JForum, it is painful to find out what have been changed/updated, especially if you have customized the html template files.
This time I found a good tool called DiffMerge to compare two versions of JForum's war files in exploded directories.
Just follow these steps:
1.You can download DiffMerge from https://sourcegear.com/diffmerge/. And install it.
2.Download jforom-2.3.4.war and jforum-2.3.5.war.
3.Uncompress these two war files by some kind of unzip tool(for example 7-zip).
4.After DiffMerge installed, execute it and use it's File>Open Folder Diff... function, and choose the jforum-2.3.4 and jforum-2.3.5 folder to compare the difference.
5.Then you could find out what have been updated by new version of JForum.

A splendid. Just what i need. I have changed a lot of stuff in my 2.1.8 version and it is time to update.