If you are an Authorized User (AU) on a CC, listen up, take note, and act.
In plain language, your credit score will drop. Maybe just a few points. Maybe a whole lot if all you have are AU TLs.
Husbands and wives, partners, parents and children, and others are legitimate AUs. There are all sorts of folks who have AU CCs. FICO's own data says 30% of consumers have AU accounts. That means tens of millions of people.
If you have an AU account, have the primary card holder contact the CCC and ask that the AU be made a joint account holder. If they want to close the account, and reopen a new joint account, tell them no. Be polite but firm in explaining you've been happy with their service, the CC has served you well, that the AU has demonstrated responsibility with the CC, and that you simply want them added as a joint card holder so their credit is not adversely harmed because of unscrupulous criminals running "Credit Rental Agencies".
They will need the AU's SSN, DOB, and perhaps a few other bits of info. They will also likely pull the AU's CRs. Unless your scores are completely trashed, there's a reasonably good chance they will convert the account. Yes, it's an inquiry on your CRs, but those don't hurt very much. Losing a good CC TL, or several, hurts much worse.
If they flat out refuse, again, do NOT let them close the account and open a new one. You might have to play hardball with them, but do it with a smile and polite firmness. Don't get nasty. Don't scream and yell. Send an Email using their secure online website, visit the bank and talk with a manager, call back and try again, ask to speak with a supervisor. Be persistent.
If they stick to their guns, and you have a balance on the CC that you can afford to PIF, pay it down to between 1 and 9 percent util--if it's not already there. If not, pay it down as much as you can. If you have available credit on another CC, do a BT. You want to make them know you are serious. Either they work with you, or you will take your CC business elsewhere.
Once the updated balances report to the CRAs, apply for a joint CC with another CCC offering good terms. You want to replace the soon to be lost AU TL with a CC TL that will report to the CRAs and will count in your FICO score. It will be new, and that hurts a little, but give it a little time and your score will bounce up.
That CCC that wouldn't work with you, retire the CC to your sock drawer. Perhaps if they see a couple months of little to no activity, they will get the hint and come around with an offer to convert the account to a joint account.
It's mid June. There is time. But don't wait until August 31. Get on the stick, today. Now.