Originally Posted by Tokyoite

If you have JGC, then best to keep it. "Leaving JGC" means cancelling your credit card and hence you lose the "almost for life" Saphire status awarded to JGC members.
You should just switch to the cheapest JGC card.
Also, althought the difference is minor, but how do you plan to requalify for Diamond for 2020? On JAL or another OW airline?

If on JAL, then the 5000 FOP bonus, the recent NRT-SEA and NRT-MNL double FOP bonuses, the domestic double FOP for a month bonus are all there to make it easier to requalify on JAL.
Furthermore, the more than double miles bonus (130%) on ALL AA/BA/IB marketed flights exclusively for JAL elites makes it easy to rack up a ton of JAL miles.

Finally, keeping the JAL card and registering for the JAL Card Tour Premium (20 USD/year) gives you 100% miles for ALL JAL flights that typically earn less than 100% (tokuwari, sakitoku, most international economy fares etc.) Another JGC perk that makes it easy to rack up a ton of JAL miles.