New Credit Card Number
After creating a backup of your Quicken data file deactivate the credit card account register (Edit Account Details, Online Services tab, Deactivate).
Close Quicken and wait a few seconds for it to finish processing.
Use the Add Accounts process ("+" icon in the Accounts Sidebar title) to locate the credit card company and the new account. Carefully LINK the new account found at the bank to the existing account register in Quicken (click on Link and be sure to select the correct account register in the popup view)
You probably will have to delete a number of downloaded transactions which have previously already been downloaded and accepted. Once that's been done, future downloads will only download new transactions.
Make sure you have no duplicates or other discrepancies.
You probably need to go into Edit Account Details, General tab and change the account register name from the old "XYZ VISA 1234" to the new name "XYZ VISA 3456" and make a note in the Comments box about the change "changed …1234 to new card number …3456, 3/30/23"1