While unashamedly an admitted novice on the keris, as an armchair student of history, I would think that given the "backend" religious factors driving the "front-end" cultural expression of the keris, it is a fair assumption the only realized difference resulting from a change in climate might have been in the materials (i.e., type of wood) used in creating the ukiran, warangka, & gambar. Snow did not keep the Hindu religion from flourishing in the Himalayas; and the same forms of weaponry with only slight-to-moderate variations may be found along a wide range of latitudes from mountain peaks of Nepal to the tropical southern subcontinent.
Of course, take my opinion for what it is worth: exactly what it cost...
Now, on the other hand, if you're speaking of how the keris may have developed had it been indigenous to Finland, I think it might have looked something like
this:
