WORKING SOLUTION!

Here is a solution that worked for me. Hopefully it will work for you.

I created a new account in QuickBooks (same type) and set up this new account with Bank Feeds. The new account now had "NotApplicable" in the grayed out Account Customer ID field. (It might have something else written for you, like your login ID, depending on your bank and type of connection).

Then, I went to the problematic account, Edit Account, looked at the Bank Feed Settings, and it miraculously already had the same thing filled in there as for the new account I just created! I went back to the General tab and changed the Account Name of the problematic account to match the new account. It asked me if I wanted to Merge and successfully allowed me to do so (it didn't allow this before).

Hooray for me, because it worked!! Hopefully, the above solution will work for you too.

If it doesn't, (I haven't looked because I didn't need to) but perhaps you can try QuickBooks' article "Solution 3: Deactivate bank feeds using Excel" or maybe try exporting all of your transactions from this account to CSV, then delete this account, then create a new one and import the transactions back (but this should be the last resort because it might mess up your reconciliations for this account and might mess up your reconciliations for other accounts as well).