You want to run two tanks of premium before switching to the premium tune because there is 87 in the fuel lines and tank. The first batch of 93 is contaminated with 87 octane fuel. If you go to the gas station when the arm is on E you still have a few gallons left. You will only be diluting it with 93 octane, not replacing it. You will also have pure 87 octane fuel in the gas lines. If you load up the the 93 performance tune and drive agressively with 87 it can very easily cause problems.
The more 93 you add the more you will dilute the 87 octane. Then when you fill your second tank you should have the 87 diluted well enough to not cause trouble.
No problem in going down, you just have to be careful going up.
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