Wicc
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Let's wait for comments from knowledgeable people regarding the version.
The save format will not change.
As for gameplay changes, there's a fix for the bug with the Satyr spell (in 1.3.5, the AI casts it on stacks where it's already applied, resulting in neutral Satyr stacks almost not attacking at all), and there might be a change to the learning specialization – ideally, nothing too drastic that would prevent you from updating and continuing your game.
Regarding critical bugs, in 1.3.5, the 2x2 mode is practically unusable – it crashes during hero exchange (fixed in 1.3.6), and in 1.3.6, another crash related to map generation has been fixed.
Also, in 1.3.6, some translations of creature/artifact names, etc., have been changed.
Otherwise, it's mostly just aesthetic fixes.
Another question is whether it might be better to start the tournament after the informative thread about the game is actually opened – but I'm not particularly well-informed on that matter.