That's actually an excellent point. We haven't yet looked at the implications of the changed options to older maps. To be honest the current living tree level was initially created to see how well the game performs with a ton of monster - and we basically are in the same state as we were for it in the original ADOM for a couple of versions: Performance is not yet where we want it to be and things will gradually improve.
So actually redesigning the forest level for non-diagonal movement probably is the best way to go and I have put it on our list t be addressed.