There are two file changes required for this:
1) In file: system/pages/createaccount.php
Replace:
PHP:
else if(!Validator::accountName($account_name))
With:
PHP:
else if(!Validator::accountId($account_name))
2) In file: tools/validate.php
Replace:
Code:
if(!Validator::accountName($account))
With:
Code:
if(!Validator::accountId($account))
You may also want to replace following in system/templates/account.create.html.twig:
Code:
<span{% if errors.account is defined %} class="red"{% endif %}>Account {% if constant('USE_ACCOUNT_NAME') %}Name{% else %}Number{% endif %}:</span>
With:
Code:
<span{% if errors.account is defined %} class="red"{% endif %}>Account Number:</span>
So it says: "Number", and no "Name".