By “half” do you mean the number of filters or their spatial size?
You could try to load the weights manually:
with torch.no_grad():
    new_model.conv.weight = old_model.conv.weight[:3]  # assuming old model used 6 filters
    # or for half the spatial size
    new_model.conv.weight = old_model.conv.weight[:, :, :2, :2]