I picked up a booking at 2pm at VLC last week that was down for 9am. They just changed it at pick-up. I am Avis preferred (only used a few times a year).
I also dropped off early due to flight/airport changes. BAH initially went to rebook this with new times (nearly 24 hours less all in all) but when repriced they said it was an extra £130 ish. Was advised to leave it by the BAH agent and just pick up late and drop off early. I don’t think it’s an issue if it’s the same day.