Diabler
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You can agree on whatever you want and however you want. On h3dm1, if it's HotA, then you can definitely upgrade vampires and place one-time boosters; if it's SoD, it's better to follow the rules of the tournament that was played on Mirror. As far as I know, there are no mandatory rules for Mirror other than the standard ХВ rules, and even then there are probably exceptions (for example, a red-rush might be allowed on L+U on Mirror if agreed upon)