Even at the highest warp speed they could manage, it still took more than one Kelvan lifespan to get from one galaxy to the other. That's why they would need a generation ship.
I think Metryq meant, why didn't the Kelvans reduce themselves to cubes for long term travel, with their equipment set to automatically restore the crew once it reached our galaxy.
The savings in life support system resources and food would be simply vast.
It would, but maybe there isn't any automatic switch on their belt gadgets? In any case, risking the whole crew in such fragile forms wouldn't be a very smart move. It takes hardly any effort at all to crush those polyhedrons.