namespace App;

-use SomeOldClass;
+use SomeNewClass;

-function someFunction(SomeOldClass $someOldClass): SomeOldClass
+function someFunction(SomeNewClass $someOldClass): SomeNewClass
 {
-    if ($someOldClass instanceof SomeOldClass) {
-        return new SomeOldClass;
+    if ($someOldClass instanceof SomeNewClass) {
+        return new SomeNewClass;
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\Renaming\Rector\Name\RenameClassRector;

return RectorConfig::configure()
    ->withConfiguredRule(RenameClassRector::class, [
        'App\\SomeOldClass' => 'App\\SomeNewClass',
    ]);

SETS:  Gmagick to Imagick phpunit/phpunit 6.0 phpunit/phpunit 8.0 symfony/* 2.7 symfony/* 3.0 symfony/* 3.3 symfony/* 4.0 symfony/* 4.1 symfony/* 4.2 symfony/* 4.3 symfony/* 4.4 symfony/* 5.0 symfony/* 5.1 symfony/* 5.3 symfony/* 5.4 symfony/* 6.0 symfony/* 6.1 symfony/* 6.2 symfony/* 6.3 symfony/* 6.4 symfony/* 7.0 SwiftMailer to Symfony Mailer doctrine/common 2.0 doctrine/dbal 3.0 doctrine/dbal 4.0 doctrine/dbal 2.11 doctrine/orm 2.13 twig/twig 1.12 twig/twig 1.40 twig/twig 2.0 twig/twig 2.40