This Help Article covers;

Normally changing the Account Name* and saving is all you need to do, however changing a Customer/Supplier Account Name* character casing from Lower to Upper Case or visa versa displays a banner saying “Account name is already in use...”

Because of restrictions on the use of Account Names, you cannot have two Account Names the same.

And because the Account Name is not case sensitive it will see “abc smith” and “Abc Smith” as being the same and display a banner following the changing of the character casing

Example: To change “abc smith” to “Abc Smith”

1. In the “Account Name*” field, enter upper case letters where you want them, then add a full stop to the Account Name “Abc Smith.”

2. [SAVE]

3. When it saves, remove the full stop

4. [SAVE] a second time