I currently hold 3 credit cards for various reasons.

Citi Dividend Card -- This was my first credit card, opened 6 years 4 months ago. It earns 1% cash back on everything plus 5% on rotating, usually pretty useless categories.

Amazon Prime Rewards Card -- This card was opened in July 2017 and earns 5% on Amazon purchases, 2% on gas and restaurants, and 1% on everything else. My wife and I make a lot purchases on Amazon because they're very convenient in the Bay Area and often the cheapest anyway.

American Express Platinum -- This card opened August 2019. This one gives 5% back on travel and 1% back on everything else, but I hold it primarily for its non-rewards travel benefits because I fly quite a bit.

I've always paid each of these cards off completely since I've opened them. They are the only lines of credit to my name. All three card providers offer FICO scores, for some reason they disagree, but depending on which site you check my credit score is 801, 791, or 762. The "tips" offered on these pages inform me that the lack of diversity to my credit lines and the youthfulness of my accounts are the biggest dings against me.

Here's what I am thinking: I am interested in getting the Fidelity credit card that requires a high credit score and gives 2% on everything. I am pretty sure I can qualify, and this way I could just put everything but Amazon.com and travel purchases on the Fidelity card. This would eliminate my current usage for the Citi card, which is where I generally just throw everything else that doesn't have an advantageous awards category on the other cards.

Paying three credit card bills already feels a little ridiculous to me, paying four would feel even more so. But I know that age of account matters for my credit score, and the Citi card is the oldest by a few years.

I am not anticipating needing additional credit until 5-6 years down the road, when I will almost certainly be moving and hopefully purchasing a first home. Part of the reason the Fidelity card appeals to me is that I could direct the rewards to a brokerage account for down payment savings! But would making a permanent change to my oldest account in my good-but-young-and-undiversified credit history cause more problems than it saves?

And before anyone says it: I know I could just leave one small monthly recurring purchase on the Citi card and set it to autopay...but if I'm misunderstanding how credit scores work and this isn't necessary, I'd like to know.