Event modding question

I am contemplating to make events more likely for a game or two just to see what happens. If I understand the guide correctly, I will have to modify XML files. I read in a different context that XML changes mean that old games become unplayable. So if I increase the probability of Events, can I do that during a game ? Only before I start a game ? Does it make old games unplayable ? Anything else I should consider ?

Don't make changes in the XML files. Make your changes in a COPY of the XML files. You can then either save the changes as a mod or, especially if you only want to make changes in a few XML lines, save the changes in the CustomAssets folder in your CIV documents folder instead of the CIV game folder. Using CustomAssets, you can easily delete the changes or modify them further and still have the unchanged game available. Changes in CustomAssets are loaded when you start a game. You can set CustomAssets to be loaded or not in the ini. file.

You can't change XML files in a game in progress because the settings get saved as part of the savegame file. If you use the CustomAssets approach, then you can set the ini. to not load CustomAssets if you want to play an old save.

Never modify the files in the main game. Always make copies and either place them in CustomAssets or make your own mod, no matter how minimal the changes. If you search the forums, you will find many instances of people asking for help because they made changes in the main game files, resulting in CIV no longer working properly or even not loading.