When you upgrade/change/switch phones, you can Smart24 onto your new phone and then request for a new access code to be sent. As long as you haven't changed your email associated to Smart24, you will be able to request a new access code without any issue.
The process for requesting a new access code is covered in this article.
If you do not receive an email with a new code, then you should contact your company's Grid Administrator to discuss next steps. If your email has changed, they will need to update this on the Grid before you can receive your new code.