You need to tell us what the swap is first.
As for switching the octane of the gas now to get ready for the new motor; you can.
It's a safe move and the price is really negligible.
If you don't, start the new swap up and let it idle for a few hours.
You can also top off the tank with E85 to boost the octane as long as you don't put in more than 3-4 gallons as E85 will degrade fuel line components.
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