I've done this before multiple times without issue. As far as your flight goes, you'll be booked in to 'G' class (for economy) which will give you fewer Avios and TP. You don't have the flexibility that you'd get with a 'plus' fare, but once or twice, when I've needed to change my flight, BA Holidays have done so as the fare class was still available on the flight I wanted to move to. If you want to pick up the car later I think they only old it for up to 2 hours (I could be wrong on that point!) but that if you are significantly delayed due to IRROPS BA Holidays will sort out the booking for you. I did read a post some time ago when someone told avis on arrival that they were going to check in to the airport hotel and collect the car in the morning and everything was fine.