Way to upgrade with not enough free ssd space?

I am asking about this for the future what I anticipate to happen.

I Lubuntu installed on an 8gb ssd and it works well, I left a bit of space to over provision which I had to reduce recently due to not enough free space for the software updater to install updates.

But this makes me think when Lubuntu 16.04 comes out I probably won't have enough space to run the automatic upgrader program.
Is there a way I can use a usb stick to store the downloads or some other method to get around this with the automatic upgrader?