Add info for mid-year Heartcore migration

umbraco-heartcore/SUMMARY.md

@@ -67,4 +67,5 @@
 
 ## Release Notes
 * [February 2024](release-notes/2024-02-releasenotes.md)
-* [April 2024](release-notes/2024-04-releasenotes.md)
+* [April 2024](release-notes/2024-04-releasenotes.md)
+* [Core Dependency Upgrade](release-notes/core-dependency-upgrade.md)

umbraco-heartcore/release-notes/core-dependency-upgrade.md

@@ -0,0 +1,45 @@
+# Heartcore Core Dependency Upgrade
+
+From July through to the end of August 2024 there is a planned major migration for all Heartcore sites created prior to 12th February 2024. This article will help you understand what to expect from it.
+
+## Benefits
+In addition to a range of quality-of-life improvements and bug fixes, after the upgrade you can expect to see the following features:
+* Support for the Block Grid Editor for managing two-dimensional content layouts.
+* Rich text editor enhancements.
+* Management API support for property type configuration.
+* Better backoffice performance across the board.
+
+For more information about these and other changes, take a look at the release notes for [February](./2024-02-releasenotes.md) and [April](./2024-04-releasenotes.md).
+
+## What to Expect
+Your project will be scheduled for upgrade in either July or August. You will receive a notification approximately two weeks prior, with the morning or afternoon of a specific day on which the migration will happen.
+
+The migration will cause a brief partial disruption to service and also contains one breaking change that will affect some sites.
+
+### During Maintenance
+You can expect the following services to be affected for approximately one hour during the migration:
+* **CMS backoffice** - Users will be unable to log in and author changes to settings, content, and media.
+* **Management API** - Applications will be unable to call Management API endpoints. This also impacts the form submission endpoint, which will return HTTP 500-series error responses during the outage window.
+
+### After Maintenence
+After the migration ***some*** non-ASCII characters in content and media URLs will be substituted with a different ASCII character sequence in the new version.
+
+{% hint style="danger" %}
+**This is a breaking change.** Unless your application has been built with redirection in mind, then external links to affected content items may no longer work post-migration.
+{% endhint %}
+
+For a non-exhaustive example, see the below table to see how some character substitutions will (or won't) change:
+
+| Character | Previous Substitution | New Substitution |
+|-----------|-----------------------|------------------|
+| å         | a                     | aa               |
+| ø         | o                     | oe               |
+| ü         | u                     | ue               |
+| æ         | ae                    | ae               |
+| ß         | ss                    | ss               |
+
+This change will only affect the URLs generated when content is published or media is saved. It will not affect property values of any content.
+
+The Redirect API will contain a reference to the pre-migration URL for each item. In order for your applications to seamlessly handle the change, it is recommended they query this API to determine if a URL has changed. If so, they should serve an HTTP 301 - Permanent Redirect response to the new URL.
+
+If your applications will be affected by this change and you need to know more, please reach out via a support ticket.