Without wishing to reopen the can of worms in the other thread, there is one exception to this. And that is when you pay the fare difference to change your Y cash ticket to a J cash ticket, which is what the OP appears to want to do (and what their screenshot from the QR website alludes to).

In that circumstance I would expect the new J fare bucket to be the lowest available on the date the change is made. But if it does not indicate which bucket when going through the process online, it may be worth calling in. The agent should be able to tell you, and offer you a higher fare bucket if preferred (for a price of course).