Adds some general information about considerations for upgrades

Author: AndyButland

16/umbraco-cms/fundamentals/setup/upgrading/README.md

@@ -8,19 +8,56 @@ In this article, you will find everything you need to upgrade your Umbraco CMS p
 
 You will find instructions on how to upgrade to a new minor or major version as well as how to run upgrades unattended.
 
+* [What is involved in an upgrade](./#what-is-involved-in-an-upgrade)
 * [Before you upgrade](./#before-you-upgrade)
 * [Upgrade to a new Major](./#upgrade-to-a-new-major)
 * [Upgrade to a new Minor](./#upgrade-to-a-new-minor)
 * [Run an unattended upgrade](./#run-an-unattended-upgrade)
 
+## What is involved in an upgrade
+
+When upgrading to a new version of Umbraco, there are four key aspects of the migration to be aware of.
+
+### Database schema and content
+
+Firstly there's the update of your Umbraco database's schema and how the content is stored.  On occasion changes are necessary to support a new feature. This might be adding new tables or columns, or updating the stored data. This is something Umbraco takes care of for you. When Umbraco starts up and is running a newer version, the necessary changes will be detected and applied.
+
+As a developer responsible for the Umbraco website, there's nothing specific you need to do here.  We will communicate key migrations that will happen on major version upgrades. As if you have a large site and significant amounts of data need to be updated, the migration could take some time to complete.
+
+Minor or patch versions may contain schema and content updates too, but they won't be extensive.
+
+### Supported features
+
+Secondly you should review your use of property editors to ensure that they are available on the new version.  It's rare for these to be removed, but it can happen when better alternatives are available.  These are only removed in major versions and with plenty of notice. They will have been indicated as legacy on earlier versions.
+
+### Project customizations
+
+The third aspect is determining and verifying that your own code customizations are compatible with the new version. This includes C# server-side functionality, Razor templates and JavaScript backoffice extensions.
+
+Extensive efforts are made to avoid breaking changes that would cause issues for these types of customization other than in a major version update.
+
+### Third-party packages
+
+Finally you should consider the packages you are using on your project. You will need to verify that the package will work with the new version of Umbraco. Or that a compatible upgrade of the package is available or planned.
+
+As above, we endeavour to avoid breaking changes that would prevent packages from working other than in a major version update.
+
+### Considerations for major version upgrades
+
+For the reasons described, projects always need to be considered case by case when upgrading to new versions.
+
+We will communicate clearly about the breaking changes and there will be extended release candidate periods to ensure upgrades can be tested and to help package developers support new major versions.
+
+We always try to minimize breaking changes but there will be cases when such updates are needed. How easy the upgrade will be depends on the breaking changes included in the major and whether your project(s) are impacted by them.
+
 ## Before you upgrade
 
 The following lists a few things to be aware of before initiating an upgrade of your Umbraco CMS project.
 
 * Sometimes, there are exceptions to general upgrade guidelines. These are listed in the [**version-specific guide**](version-specific/). Be sure to read this article before moving on.
 * Ensure your setup meets the [requirements](../requirements.md) for the new versions you will be upgrading your project to.
 * Things may go wrong for different reasons. Be sure to **always** keep a backup of both your site's files and the database. This way, you can always return to a version that you know works.
-* Before upgrading to a new major version, check if the packages you're using are compatible with the version you're upgrading to. On the package's download page, in the **Project compatibility** area, click **View details** to check version-specific compatibility.
+* Before upgrading to a new major version, check if the packages you're using are compatible with the version you're upgrading to. On the package's page on the [Umbraco Marketplace](https://marketplace.umbraco.com/), check the "Umbraco versions" field.
 
 {% hint style="info" %}
 It is necessary to run the upgrade installer on each environment of your Umbraco site. This means that you need to repeat the steps below on each of your environments in order to complete the upgrade.