Is your current reservation for returning the car at the same hour as for picking it up? If so, all that is written above certainly applies.

The only case where it might not apply is if you originally said you'd be renting for 3.5 days, and they rounded that up to 4 days, and now you're turning it into 3.7 days, which should still round up to 4 days. (However, the fact that it's returned late may cause the whole 4 days to be recomputed at a last-minute rate, which itself could cost more. So it's no guarantee that changing it from 3.5 days to 3.7 days would not cost much. It's just that there's some hope in that case, whereas in changing it from 3 days 0 hours to 3 days 3 hours I don't see any hope.

So please explain exactly how long (down to the hour) your current reservation is.