It's on-nigh impossible to make a bike tyre aquaplane so tread isn't needed on normal roads. On normal roads when you can see tyre carcass or you start getting frequent punctures change the tyre. If you're riding on lose, soft or hard pack surfaces which need some kind of tread to give grip then play it by ear (if you feel like you're loosing control of the bike to easily etc). In either scenario when replacing a tyre you put the new tyre on the front wheel & if that means moving the front tyre on the rear-wheel then so be it.