There are many reasons someone might want to move a credit line between accounts. Perhaps the most common situation is when closing a card to avoid its annual fee; rather than seeing your total available credit decrease by the amount of the card's credit limit, you can transfer all but (typically) a nominal sum to another credit card issued by the same bank.
Alternatively, if you have a large credit line on a non-rewards-earning credit card like the Chase Slate from your pre-travel-hacking days, then you might decide to transfer the credit line to a more lucrative card also issued by Chase.
Each bank has a different procedure for this operation. In my experience, I've been able to transfer all but $2,500 from my Chase credit cards when closing them, and I've done that both over the phone and through the Secure Message Center without any difficulty.
Bank of America, on the other hand, in my experience requires a hard credit pull when transferring credit lines – even though they're not extending any additional credit!
American Express has fiddled with their procedure over the years, but since I just did this, I thought I'd share the method that worked for me today.
What cards are eligible?
American Express's basic rule is that a card must be open for at least 12 months before you can transfer that card's credit line away from it. In theory, credit lines can be transferred to a credit card almost immediately after the card account is opened.
I can now report from personal experience that the 12 month clock does not reset when you upgrade a credit card: I upgraded my Hilton HHonors Surpass card less than 12 months ago, but was able to reallocate credit away from it today.
Additionally, credit lines cannot be reallocated from business credit cards to consumer credit cards. Any other combination is theoretically possible using the online system: from consumer to business, between business, and between consumer credit cards.
Avoid the phone and online agents
I twice attempted exactly the same procedure that was successful today, once over the phone and once through the online messaging system, but was unsuccessful both times. Both times I believe the agents attempted to reallocate the credit lines from my (new) Blue Cash card to my HHonors card, instead of the other way around. I may have simply had incompetent agents helping me both times, or they might all be equally poorly-trained.
So for the sake of your sanity, use the online system if possible.
How it works
From your American Express home page, click on "Profile" near the top of the screen, then "Manage Credit Limit:"
From there, look for "Transfer Available Credit to Another Card," and click "Start:"
From there, select the card you want to transfer your available credit limit from:
And the card you want to transfer your available credit to:
Once you confirm the request, you'll be immediately notified whether the request was successful or not, and your available credit limits will almost immediately reflect the change (they say it can take up to 15 minutes).