4th Edition Rules / 3rd Edition Scenarios

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If you play with different rules than were intended to be played with a scenario, you are playing a different scenario. I will always play with the rules that were contemporary to the module for this reason, be that 2nd, 3rd, or 4th edition. For a lot of modules, the differences in terrain rules alone change so much you would impact the scenario balance less by adding a full platoon of King Tigers to the Germans in every scenario. 

The dangers of house and optional rules have been discussed to death, but retroactively changing everything in an old scenario is much the same.