Just be sure to run your car till there is REALLY low fuel, I'm not sure but I thought mixing gases wasn't such a great idea. You could always draint he tank but that takes time as well. It's up to you. I had 87 octane the first time I filled my gts and I didn't notice any knocking, but then again I didn't drive it hard.
This is a bad idea. Draining the tank down low on poor grade gas could cause problems with your injectors.
Best bet is to run it down to 1/2 of a tank, then fill it with premium.
then down to a 1/4 tank, fill with premium, then to a 1/4 tank again and fill with premium.
then, only a small amount of the gas in the tank will be 89.
It's not harmful, whatever you do don't rev it up or it'll knock and ping