For anyone trying to use the 2.5.3 uninstall script to move up to 3.0.0, there is a problem with the uninstall script:

Around line 36 in /YOUR_ADMIN/includes/init_includes/init_local_sales_taxes_uninstall.php:

PHP Code:
if(defined(MODULE_ORDER_TOTAL_COUNTY_LOCAL_TAX_STATUS)) { 
Replace with :

PHP Code:
if(defined('MODULE_ORDER_TOTAL_COUNTY_LOCAL_TAX_STATUS')) { 
This should clear the 500 server errors that pop up.