Any discussion about Octane needs to involve what octane, do you mean RON, MON or AKI ?. The operators manual has very clear requirements, I suggest yo do not go outside them. There is also a Rotax document on ethanol percentages. There is little downside in using a higher octane fuel than specified. Yes "old" fuel should be pumped out and into your car and new fuel added but what is old ?
Three months six months ?