Your old license code may have been entered into config.php. This overrides codes entered through the web interface.

Check config.php (in the directory where you installed Deskpro. You will need access to the server file system to do this).

If there's a section like this at the end:

$DP_LIC_STR = <<<DP_LIC_STR

old code

DP_LIC_STR;

define('DP_LIC_STR', $DP_LIC_STR);

Delete it or enter the new code there.