OP, IMHO you should think of the price gap as a benefit in this case. If you buy a flex fare, you're playing Aerolotto but spending way more money. Your chances of getting the upgrade on this route is very low; more likely you'd pay a more expensive fare and end up sitting in the same Y seat.

If you buy a Standard fare, you'll pretty much have the same chance of an upgrade, but you'll only pay the more expensive co-pay (which is only $100 more than flex IIRC) if your upgrade actually clears. IMHO you'll be better off buying the standard fare.

If you do want to pay more, it looks like on several dates Premium Economy (Lowest) is cheaper than Flex. You might want to go that route; guarantee yourself premium economy and have a higher chance at J upgrade.