Neither Yokoso nor Explorer pass can pre-book class j upgrade.

Both of them needs to wait until check-in...so in terms of upgrade to class j / f, there's no differences.

Here is an extreme example of what I meant by maximizing chances of scoring class j / f.

You book the last HND-CTS flight by using Yokoso, and go to check-in counter before first flight check-in starts, as long as there are J/F seats available that day, you can change your flight to the flights that has availability. Whereas explorer pass holder doesn't have such flexibility.