Both of my CapOne cards are same age, Plat is just a month older. I have 3 cards and once the merge take effects, I believe it will affect my credit score since I have been doing AZEO.
If there's an effect on your score, it'd be for a slightly reduced number of open accounts vs. accounts reporting postive balances. If that happens, a dip will be minimal, maybe a point or two.
I think you're set on this, but make sure you're not "inventing" ways to spend in order to meet sign-up bonuses. If it can help, temporarily shift some spending that would normally go on another card.
Also, if you're currently paying interest on your BoA card, you'll want to be sure that your grace period is reset. Ask us for help if this is relevant and you need some instructions.