That's a great question that I don't know the answer to. My best guess is that the points would be separately maintained, and would be treated essentially as two different currencies. Would be great to hear from anyone who has had both card types simultaneously.
I only have the Arrival Premier, never had the Arrival/Arrival Plus. From a purely logical viewpoint, if you ultimately plan on getting the Premier, you should probably wrap up your current card and then move forward with the new one. I highly doubt they will ever change the carry-over policy, and besides credit score considerations (aveage age of credit, hard pulls, etc.), I can't see any reason to hold both. If the Premier airline partners work for you and transferring interests you, it's great to have the best of both worlds (transfer or use as simple statement credit - you pick). Also to consider is the Premier's $15K/$25K bonus milestones, if you have the ability to reach them: 25K bonus miles.
I'm very satisfied with the Arrival Premier and it will definitely be a keeper for me, but again, it all depends on your personal circumstances (this is coming from soneone who holds other premium cards from the big issuers). If they would allow free airport lounge access for the primary cardholder (ideally with free guesting privileges) instead of the current pay-to-access, I would even consider dropping other cards in my wallet.
Update: It now looks like not only is upgrading out of the question, but the Arrival Premier is now closed to new applications entirely. I just started a new thread in this forum to address and discuss this.
They really screwed up with this roll out... I gave up already and switch to sapphire reserve. I’m guessing a lot of other people are doing/have done the same.
It was definitely a gamble. Depending on perspective, it could be called innovative or unadvisable. Interesting thing is that Barclays has said that part of their motivation was that consumers had told them that they wanted, and would pay for, a card with ongoing reward bonuses like this.