People's plans change all the time. I don't think you'd have any trouble.
But if you go in with the plan of day hiking, you should try to change the permit and get the Day Use Mt Whitney Zone permit. It's the same as Main Trail day hikers need. You can very likely get the day use permit, as there are always no-shows and group dropouts.
Biggest part is making sure you have the parking pass for Whitney Portal.
