|The tank as empty as possible. And then fill up with whatever fuel you are using and switch the tune.
I have dual widebands in my car and know enough about tuning and where things should be in order to make sure everything is safe. Say that you’ve been running e85 and you have a quarter of a tank and can only fill up with 93, you can switch to your 93 tune, and dial boost down. The car will run leaner with the higher ethanol content but this is why you dial down boost.
Say you are running your 93 tune and fill up with e85, depending on how much e85 you have in the tank, you can still technically run your e85 tune or your 93. Except you need to watch what you are boosting at again. And watch your AFRs.
As long as your fueling mods are in order, and you have a chip tune you can fill up with a mixture of e85, people have done this for years. The car runs cleaner, and smoother. You can even up the boost a little bit if you wanted to. E85 is magic, but the real gains down start to come until you start tweaking with timing, bc it won’t knock, you’ll throw a rod before.