If I upgrade a Y Flex mileage ticket to J using miles, if F subsequently becomes available can I modify the existing booking to F for only the difference in award price between the miles I've used so far to the cost of an F award or do I need to cancel the entire reservation (I assume upgrade awards are fully cancellable just like my ticket), wait until the miles come back into my account and then book the F award (assuming it's still available....)?

And how long does it usually take for the miles to be redeposited?