These two questions from the FAQ in the thread summary should address your concerns. Only you can decide if it's worth it to cancel now and re-up for 3 years, or just doing an EA Play 1 year extension, or taking the gamble on waiting till expiry:
Q: I'm currently a Gamepass Ultimate subscriber till dd/mm/yyyy. Is there any way for me to extend my Ultimate subscription for cheap?
A: You can't use Live Gold codes to extend your Ultimate subscription at 1:1 if you're a current subscriber, unfortunately. They convert it at MSRP. So at the regular price of your 12 month code ($70), they'll give you $70/$16.99 = 4 months. The same conversion process works for 6 and 3 month codes, too. If the end of your subscription is in the near future (within a few months), you can just cancel it outright and start over. You lose your remaining months, but you can at least re-stack for 3 full years if that's your thing. If the end of your subscription is years in the future, your only real (cheap) option to extend is to go the EA Play conversion route. See this thread for more details: microsoft-ea-access-play-merge-game-pas ... h-2401871/
Q: When does this expire? Should I wait till I get my Xbox Series X/S before doing this?
A: Nobody knows when this will expire. XBox Live Gold is rumoured to be going away with the next gen, due to Halo 5 announcing free online play and Microsoft no longer offering 12 month Live Gold subscriptions on their website/consoles (but existing 12 month subscribers with auto-renew are still renewing for 12 months). In theory, if those rumours are true, then in theory we should be able to do this trick until the next gen consoles launch on Nov 10, 2020. If it's false and Live Gold isn't going anywhere, then there's no way to know when (or if) Microsoft will ever stop converting at 1:1.