diff --git a/README.md b/README.md
index 5b57997..62e17be 100644
--- a/README.md
+++ b/README.md
@@ -2,12 +2,12 @@
 
 [![Latest Stable Version](https://img.shields.io/packagist/v/lmc/coding-standard.svg?style=flat-square)](https://packagist.org/packages/lmc/coding-standard)
 
-PHP coding standard used in [Alma Career Czechia](https://www.almacareer.com/) (formerly LMC) products.
+PHP coding standard used in [Alma Career Czechia][Alma Career] (formerly LMC) products.
 
-Standard is based on [PSR-12](https://www.php-fig.org/psr/psr-12/) and adds
+The standard is based on [PSR-12][psr-12] and partially [PER 2.0][per-2] and adds
 various checks to make sure the code is readable, follows the same conventions, and does not contain common mistakes.
 
-We use [EasyCodingStandard] to define and execute checks created for both [PHP-CS-Fixer] and [PHP_CodeSniffer].
+We use [EasyCodingStandard][ecs] to define and execute checks created for both [PHP-CS-Fixer] and [PHP_CodeSniffer].
 
 ## Installation
 
@@ -59,7 +59,7 @@ Now you will be able to run the fix using `composer analyze` and execute automat
 
 ### Add custom checks or override default settings
 
-On top of default code-style rules you are free to add any rules from [PHP-CS-Fixer] or [PHP_CodeSniffer].
+On top of the default code-style rules, you are free to add any rules from [PHP-CS-Fixer] or [PHP_CodeSniffer].
 If needed, you can also override some default settings.
 
 ```php
@@ -83,12 +83,12 @@ return ECSConfig::configure()
     /* (...) */
 ```
 
-See [EasyCodingStandard docs](https://github.com/symplify/easy-coding-standard#configuration) for more configuration options.
+See [EasyCodingStandard docs][ecs-docs] for more configuration options.
 
 
 ### Exclude (skip) checks or files
 
-You can configure your `ecs.php` to entirely skip some files, disable specific checks or suppress specific errors.
+You can configure your `ecs.php` file to entirely skip some files, disable specific checks, or suppress specific errors.
 
 ```php
 <?php declare(strict_types=1);
@@ -114,18 +114,23 @@ return ECSConfig::configure()
     /* (...) */
 ```
 
-See [EasyCodingStandard docs](https://github.com/symplify/easy-coding-standard#configuration) for more configuration options.
+See [EasyCodingStandard docs][ecs-docs] for more configuration options.
 
 ### IDE integration
 
-For integration with PHPStorm etc. follow instructions in EasyCodingStandard [README](https://github.com/symplify/easy-coding-standard#your-ide-integration).
+For integration with PHPStorm and other IDEs, follow instructions in [EasyCodingStandard README][ecs-readme-ide].
 
 ## Changelog
-For the latest changes see [CHANGELOG.md](CHANGELOG.md) file. We follow [Semantic Versioning](https://semver.org/).
+For the latest changes, see [CHANGELOG.md](CHANGELOG.md) file. This library follows [Semantic Versioning](https://semver.org/).
 
 ## License
-This library is open source software licensed under the [MIT license](LICENSE.md).
+This library is open-source software licensed under the [MIT license](LICENSE.md).
 
+[Alma Career]: https://www.almacareer.com/
 [PHP-CS-Fixer]: https://github.com/FriendsOfPHP/PHP-CS-Fixer
 [PHP_CodeSniffer]: https://github.com/squizlabs/PHP_CodeSniffer
-[EasyCodingStandard]: https://github.com/symplify/easy-coding-standard
+[psr-12]: https://www.php-fig.org/psr/psr-12/
+[per-2]: https://www.php-fig.org/per/coding-style/
+[ecs]: https://github.com/easy-coding-standard/easy-coding-standard
+[ecs-docs]: https://github.com/easy-coding-standard/easy-coding-standard#configure
+[ecs-readme-ide]: https://github.com/easy-coding-standard/easy-coding-standard/blob/9.0.0/README.md#your-ide-integration
diff --git a/UPGRADE-4.0.md b/UPGRADE-4.0.md
index 48b1a87..e614b51 100644
--- a/UPGRADE-4.0.md
+++ b/UPGRADE-4.0.md
@@ -1,7 +1,7 @@
 # Upgrading from 3.x to 4.0
 
 ### 1. Update dependency in composer.json
-In require-dev section change the version constraint:
+In the `require-dev` section of `composer.json` change the version constraint:
 
 ```diff
 -        "lmc/coding-standard": "^3.3",
@@ -11,9 +11,9 @@ In require-dev section change the version constraint:
 Then run `composer update`.
 
 ### 2. Configuration updates
-Configuration now uses `ECSConfig` class instead of `ContainerConfigurator`.
+The configuration now uses `ECSConfig` class instead of `ContainerConfigurator`.
 
-Update your `ecs.php` to use the new configuration style:
+Update your `ecs.php` file to use the new configuration style:
 
 ```diff
 -use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
@@ -85,14 +85,27 @@ $ vendor/bin/ecs check --ansi src/ tests/ # old
 $ vendor/bin/ecs check --ansi # new
 ```
 
-### 5. Sanity check
-Besides running your code style checks, you can ensure all predefined checks are loaded as well, by running:
+### 5. BE CAREFUL WITH SUGGESTED CHANGES! ⚠️
+
+Some of the new default fixers introduced in php-coding-standard 4.0 and 4.1 suggest changes, which - if not
+thoughtfully reviewed - can change the code behavior. Especially changes introduced by (but not limited to!):
+
+- PhpdocToPropertyTypeFixer + PropertyTypeHintSniff
+- PhpdocToParamTypeFixer + ParameterTypeHintSniff
+- PhpdocToReturnTypeFixer + ReturnTypeHintSniff
+
+**Always carefully review the changes suggested by all fixers!** You may want to skip some of the checks
+(using `withSkip()`) in the first phase of upgrading to the new version of the coding standard
+or you can introduce some of the rules gradually or on a file-by-file basis.
+
+### 6. Sanity check
+Besides running your code style checks, you can ensure all predefined checks are loaded by running:
 
 ```sh
 vendor/bin/ecs list-checkers
 ```
 
-The result should end with something like:
+The result should end with something like this:
 ```
  41 checkers from PHP_CodeSniffer:
  ...